diff --git a/backend/alembic.ini b/backend/alembic.ini
new file mode 100644
index 0000000..60aab67
--- /dev/null
+++ b/backend/alembic.ini
@@ -0,0 +1,88 @@
+# Alembic Configuration File
+
+[alembic]
+# path to migration scripts
+script_location = alembic
+
+# template used to generate migration file names; the default
+# file_template = %%(rev)s_%%(slug)s
+
+# sys.path path, will be prepended to sys.path if present.
+# defaults to the current working directory.
+prepend_sys_path = .
+
+# timezone to use when rendering the date within the migration file
+# as well as the filename.
+# timezone =
+
+# max length of characters to apply to the "slug" field
+# truncate_slug_length = 40
+
+# set to 'true' to run the environment during
+# the 'revision' command, regardless of autogenerate
+# revision_environment = false
+
+# set to 'true' to allow .pot files to be overwritten
+# generate_empty_script = false
+
+# python function to use for producing template names
+# file_template = %%(rev)s_%%(slug)s
+
+# the output encoding used when revision files
+# are written from script.py.mako
+# output_encoding = utf-8
+
+sqlalchemy.url = sqlite:///./data/app.db
+
+
+[post_write_hooks]
+# post_write_hooks defines scripts or Python functions that are run
+# on newly generated revision scripts.  See the documentation for further
+# detail and examples
+
+# format using "black" - use the console_scripts runner, against the "black" entrypoint
+# hooks = black
+# black.type = console_scripts
+# black.entrypoint = black
+# black.options = -l 79
+
+# lint with attempts to fix using "ruff" - use the exec runner, execute a binary
+# hooks = ruff
+# ruff.type = exec
+# ruff.executable = %(here)s/.venv/bin/ruff
+# ruff.options = check --fix REVISION_SCRIPT_FILENAME
+
+# Logging configuration
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARN
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARN
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S
diff --git a/backend/alembic/env.py b/backend/alembic/env.py
new file mode 100644
index 0000000..ffff025
--- /dev/null
+++ b/backend/alembic/env.py
@@ -0,0 +1,78 @@
+"""Alembic 环境配置。
+
+import app.models 以注册所有 ORM 模型到 Base.metadata，
+使 alembic revision --autogenerate 能检测模型变更。
+"""
+
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy import engine_from_config, pool
+
+from app.core.config import get_settings
+from app.models import Base  # noqa: F401 — 触发全部模型注册
+
+# this is the Alembic Config object
+config = context.config
+
+# 根据应用配置覆盖 sqlalchemy.url
+settings = get_settings()
+config.set_main_option("sqlalchemy.url", settings.database_url)
+
+# Interpret the config file for Python logging.
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# target metadata for autogenerate
+target_metadata = Base.metadata
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL
+    and not an Engine, though an Engine is acceptable
+    here as well.  By skipping the Engine creation
+    we don't even need a DBAPI to be available.
+
+    Calls to context.execute() here emit the given string to the
+    script output.
+    """
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    """Run migrations in 'online' mode.
+
+    In this scenario we need to create an Engine
+    and associate a connection with the context.
+    """
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection,
+            target_metadata=target_metadata,
+        )
+
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()
diff --git a/backend/alembic/script.py.mako b/backend/alembic/script.py.mako
new file mode 100644
index 0000000..958df87
--- /dev/null
+++ b/backend/alembic/script.py.mako
@@ -0,0 +1,25 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}
diff --git a/backend/alembic/versions/.gitkeep b/backend/alembic/versions/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/backend/alembic/versions/5c27c8e31fde_initial_schema.py b/backend/alembic/versions/5c27c8e31fde_initial_schema.py
new file mode 100644
index 0000000..0fc6481
--- /dev/null
+++ b/backend/alembic/versions/5c27c8e31fde_initial_schema.py
@@ -0,0 +1,142 @@
+"""initial_schema
+
+Revision ID: 5c27c8e31fde
+Revises: 
+Create Date: 2026-09-01 12:22:22.337100
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '5c27c8e31fde'
+down_revision: Union[str, None] = None
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('plans',
+    sa.Column('code', sa.String(length=32), nullable=False, comment='套餐代码 (free/basic/pro)'),
+    sa.Column('name', sa.String(length=64), nullable=False, comment='套餐名称'),
+    sa.Column('storage_quota', sa.Integer(), nullable=False, comment='存储配额 (字节)'),
+    sa.Column('max_file_size', sa.Integer(), nullable=False, comment='单文件大小上限 (字节)'),
+    sa.Column('is_active', sa.Boolean(), nullable=False, comment='是否可用'),
+    sa.Column('id', sa.String(length=32), nullable=False, comment='UUID4 十六进制主键'),
+    sa.Column('created_at', sa.String(length=32), nullable=False, comment='创建时间 (ISO8601)'),
+    sa.Column('updated_at', sa.String(length=32), nullable=False, comment='更新时间 (ISO8601)'),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('code')
+    )
+    op.create_table('users',
+    sa.Column('username', sa.String(length=64), nullable=False, comment='用户名'),
+    sa.Column('email', sa.String(length=255), nullable=False, comment='邮箱'),
+    sa.Column('password_hash', sa.String(length=255), nullable=False, comment='Argon2id 密码哈希'),
+    sa.Column('status', sa.String(length=16), nullable=False, comment='状态 (active/disabled)'),
+    sa.Column('plan_id', sa.String(length=32), nullable=False, comment='套餐 ID'),
+    sa.Column('storage_used', sa.Integer(), nullable=False, comment='已用存储 (字节)'),
+    sa.Column('id', sa.String(length=32), nullable=False, comment='UUID4 十六进制主键'),
+    sa.Column('created_at', sa.String(length=32), nullable=False, comment='创建时间 (ISO8601)'),
+    sa.Column('updated_at', sa.String(length=32), nullable=False, comment='更新时间 (ISO8601)'),
+    sa.ForeignKeyConstraint(['plan_id'], ['plans.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_users_email'), 'users', ['email'], unique=True)
+    op.create_index(op.f('ix_users_username'), 'users', ['username'], unique=True)
+    op.create_table('knowledge_bases',
+    sa.Column('user_id', sa.String(length=32), nullable=False, comment='所有者用户 ID'),
+    sa.Column('name', sa.String(length=255), nullable=False, comment='知识库名称'),
+    sa.Column('description', sa.Text(), nullable=True, comment='知识库描述'),
+    sa.Column('enabled', sa.Boolean(), nullable=False, comment='是否启用'),
+    sa.Column('token_hash', sa.String(length=64), nullable=False, comment='SHA-256(secret_token) 十六进制'),
+    sa.Column('token_encrypted', sa.Text(), nullable=True, comment='Fernet 加密的 token 原文'),
+    sa.Column('token_hint', sa.String(length=16), nullable=True, comment='token 末 8 位明文，供后台识别'),
+    sa.Column('id', sa.String(length=32), nullable=False, comment='UUID4 十六进制主键'),
+    sa.Column('created_at', sa.String(length=32), nullable=False, comment='创建时间 (ISO8601)'),
+    sa.Column('updated_at', sa.String(length=32), nullable=False, comment='更新时间 (ISO8601)'),
+    sa.ForeignKeyConstraint(['user_id'], ['users.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_knowledge_bases_token_hash'), 'knowledge_bases', ['token_hash'], unique=True)
+    op.create_index(op.f('ix_knowledge_bases_user_id'), 'knowledge_bases', ['user_id'], unique=False)
+    op.create_table('document_categories',
+    sa.Column('knowledge_base_id', sa.String(length=32), nullable=False, comment='所属知识库 ID'),
+    sa.Column('name', sa.String(length=255), nullable=False, comment='分类名称'),
+    sa.Column('sort_order', sa.Integer(), nullable=False, comment='排序序号'),
+    sa.Column('id', sa.String(length=32), nullable=False, comment='UUID4 十六进制主键'),
+    sa.Column('created_at', sa.String(length=32), nullable=False, comment='创建时间 (ISO8601)'),
+    sa.Column('updated_at', sa.String(length=32), nullable=False, comment='更新时间 (ISO8601)'),
+    sa.ForeignKeyConstraint(['knowledge_base_id'], ['knowledge_bases.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_document_categories_knowledge_base_id'), 'document_categories', ['knowledge_base_id'], unique=False)
+    op.create_table('documents',
+    sa.Column('knowledge_base_id', sa.String(length=32), nullable=False, comment='所属知识库 ID'),
+    sa.Column('user_id', sa.String(length=32), nullable=False, comment='所有者用户 ID (冗余，便于隔离校验)'),
+    sa.Column('category_id', sa.String(length=32), nullable=True, comment='分类 ID'),
+    sa.Column('original_filename', sa.String(length=512), nullable=False, comment='原始文件名 (仅展示)'),
+    sa.Column('storage_path', sa.String(length=1024), nullable=False, comment='相对于 data/ 的物理存储路径'),
+    sa.Column('markdown_path', sa.String(length=1024), nullable=True, comment='相对于 data/ 的 Markdown 文件路径'),
+    sa.Column('file_size', sa.Integer(), nullable=False, comment='文件大小 (字节)'),
+    sa.Column('mime_type', sa.String(length=127), nullable=False, comment='MIME 类型'),
+    sa.Column('file_ext', sa.String(length=16), nullable=False, comment='文件扩展名 (.docx/.pdf)'),
+    sa.Column('sha256', sa.String(length=64), nullable=False, comment='文件 SHA-256 哈希'),
+    sa.Column('doc_token_hash', sa.String(length=64), nullable=True, comment='SHA-256(document_token) 十六进制'),
+    sa.Column('doc_token_encrypted', sa.Text(), nullable=True, comment='Fernet 加密的 document_token 原文'),
+    sa.Column('doc_token_hint', sa.String(length=16), nullable=True, comment='document_token 末 8 位明文'),
+    sa.Column('title', sa.String(length=512), nullable=True, comment='文档标题 (用户可改)'),
+    sa.Column('description', sa.Text(), nullable=True, comment='文档描述 (用户可改)'),
+    sa.Column('keywords', sa.Text(), nullable=True, comment='关键词 (逗号分隔)'),
+    sa.Column('content_summary', sa.Text(), nullable=True, comment='抽取式摘要 (~200字)'),
+    sa.Column('status', sa.String(length=16), nullable=False, comment='状态 (PENDING/PROCESSING/READY/FAILED/DELETED)'),
+    sa.Column('error_code', sa.String(length=64), nullable=True, comment='错误码 (如 SCANNED_PDF_NO_TEXT_LAYER)'),
+    sa.Column('id', sa.String(length=32), nullable=False, comment='UUID4 十六进制主键'),
+    sa.Column('created_at', sa.String(length=32), nullable=False, comment='创建时间 (ISO8601)'),
+    sa.Column('updated_at', sa.String(length=32), nullable=False, comment='更新时间 (ISO8601)'),
+    sa.ForeignKeyConstraint(['category_id'], ['document_categories.id'], ),
+    sa.ForeignKeyConstraint(['knowledge_base_id'], ['knowledge_bases.id'], ),
+    sa.ForeignKeyConstraint(['user_id'], ['users.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_documents_doc_token_hash'), 'documents', ['doc_token_hash'], unique=True)
+    op.create_index(op.f('ix_documents_knowledge_base_id'), 'documents', ['knowledge_base_id'], unique=False)
+    op.create_index(op.f('ix_documents_user_id'), 'documents', ['user_id'], unique=False)
+    op.create_table('access_logs',
+    sa.Column('knowledge_base_id', sa.String(length=32), nullable=False, comment='知识库 ID'),
+    sa.Column('document_id', sa.String(length=32), nullable=True, comment='文档 ID (可选)'),
+    sa.Column('path', sa.String(length=1024), nullable=False, comment='请求路径'),
+    sa.Column('accessed_at', sa.String(length=32), nullable=False, comment='访问时间 (ISO8601)'),
+    sa.Column('user_agent', sa.Text(), nullable=True, comment='User-Agent'),
+    sa.Column('request_type', sa.String(length=32), nullable=True, comment='请求类型 (html/md/txt/json/search)'),
+    sa.Column('id', sa.String(length=32), nullable=False, comment='UUID4 十六进制主键'),
+    sa.Column('created_at', sa.String(length=32), nullable=False, comment='创建时间 (ISO8601)'),
+    sa.Column('updated_at', sa.String(length=32), nullable=False, comment='更新时间 (ISO8601)'),
+    sa.ForeignKeyConstraint(['document_id'], ['documents.id'], ),
+    sa.ForeignKeyConstraint(['knowledge_base_id'], ['knowledge_bases.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_access_logs_knowledge_base_id'), 'access_logs', ['knowledge_base_id'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_access_logs_knowledge_base_id'), table_name='access_logs')
+    op.drop_table('access_logs')
+    op.drop_index(op.f('ix_documents_user_id'), table_name='documents')
+    op.drop_index(op.f('ix_documents_knowledge_base_id'), table_name='documents')
+    op.drop_index(op.f('ix_documents_doc_token_hash'), table_name='documents')
+    op.drop_table('documents')
+    op.drop_index(op.f('ix_document_categories_knowledge_base_id'), table_name='document_categories')
+    op.drop_table('document_categories')
+    op.drop_index(op.f('ix_knowledge_bases_user_id'), table_name='knowledge_bases')
+    op.drop_index(op.f('ix_knowledge_bases_token_hash'), table_name='knowledge_bases')
+    op.drop_table('knowledge_bases')
+    op.drop_index(op.f('ix_users_username'), table_name='users')
+    op.drop_index(op.f('ix_users_email'), table_name='users')
+    op.drop_table('users')
+    op.drop_table('plans')
+    # ### end Alembic commands ###
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index e69de29..3b5dffc 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -0,0 +1,21 @@
+"""SQLAlchemy 2.0 模型包。"""
+
+from app.models.base import Base, generate_uuid, utcnow_iso
+from app.models.plan import Plan
+from app.models.user import User
+from app.models.knowledge_base import KnowledgeBase
+from app.models.document_category import DocumentCategory
+from app.models.document import Document
+from app.models.access_log import AccessLog
+
+__all__ = [
+    "Base",
+    "generate_uuid",
+    "utcnow_iso",
+    "Plan",
+    "User",
+    "KnowledgeBase",
+    "DocumentCategory",
+    "Document",
+    "AccessLog",
+]
diff --git a/backend/app/models/access_log.py b/backend/app/models/access_log.py
new file mode 100644
index 0000000..6fd4998
--- /dev/null
+++ b/backend/app/models/access_log.py
@@ -0,0 +1,56 @@
+"""AccessLog 访问日志模型。"""
+
+from sqlalchemy import ForeignKey, Integer, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class AccessLog(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """访问日志模型。
+
+    技术审查 §2.2：access_logs 表。
+    """
+
+    __tablename__ = "access_logs"
+
+    knowledge_base_id: Mapped[str] = mapped_column(
+        String(32),
+        ForeignKey("knowledge_bases.id"),
+        nullable=False,
+        index=True,
+        comment="知识库 ID",
+    )
+    document_id: Mapped[str | None] = mapped_column(
+        String(32),
+        ForeignKey("documents.id"),
+        nullable=True,
+        comment="文档 ID (可选)",
+    )
+    path: Mapped[str] = mapped_column(
+        String(1024),
+        nullable=False,
+        comment="请求路径",
+    )
+    accessed_at: Mapped[str] = mapped_column(
+        String(32),
+        nullable=False,
+        comment="访问时间 (ISO8601)",
+    )
+    user_agent: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="User-Agent",
+    )
+    request_type: Mapped[str | None] = mapped_column(
+        String(32),
+        nullable=True,
+        comment="请求类型 (html/md/txt/json/search)",
+    )
+
+    # 关系
+    knowledge_base = relationship("KnowledgeBase", back_populates="access_logs", lazy="selectin")
+    document = relationship("Document", back_populates="access_logs", lazy="selectin")
+
+    def __repr__(self) -> str:
+        return f"<AccessLog path={self.path!r} at={self.accessed_at!r}>"
diff --git a/backend/app/models/base.py b/backend/app/models/base.py
new file mode 100644
index 0000000..3ae2ec2
--- /dev/null
+++ b/backend/app/models/base.py
@@ -0,0 +1,52 @@
+"""SQLAlchemy 2.0 基础模型类与 Mixin。"""
+
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import String, text
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
+
+
+def generate_uuid() -> str:
+    """生成 UUID4 十六进制字符串（32字符）。"""
+    return uuid.uuid4().hex
+
+
+def utcnow_iso() -> str:
+    """返回 UTC 当前时间的 ISO8601 字符串。"""
+    return datetime.now(timezone.utc).isoformat()
+
+
+class Base(DeclarativeBase):
+    """SQLAlchemy 声明式基类。"""
+    pass
+
+
+class UUIDPrimaryKeyMixin:
+    """UUID 主键 Mixin。"""
+
+    id: Mapped[str] = mapped_column(
+        String(32),
+        primary_key=True,
+        default=generate_uuid,
+        comment="UUID4 十六进制主键",
+    )
+
+
+class TimestampMixin:
+    """创建/更新时间 Mixin。"""
+
+    created_at: Mapped[str] = mapped_column(
+        String(32),
+        default=utcnow_iso,
+        nullable=False,
+        comment="创建时间 (ISO8601)",
+    )
+
+    updated_at: Mapped[str] = mapped_column(
+        String(32),
+        default=utcnow_iso,
+        onupdate=utcnow_iso,
+        nullable=False,
+        comment="更新时间 (ISO8601)",
+    )
diff --git a/backend/app/models/document.py b/backend/app/models/document.py
new file mode 100644
index 0000000..584d969
--- /dev/null
+++ b/backend/app/models/document.py
@@ -0,0 +1,128 @@
+"""Document 文档模型。"""
+
+from sqlalchemy import ForeignKey, Integer, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class Document(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """文档模型。
+
+    技术审查 §2.2：documents 表。
+    """
+
+    __tablename__ = "documents"
+
+    knowledge_base_id: Mapped[str] = mapped_column(
+        String(32),
+        ForeignKey("knowledge_bases.id"),
+        nullable=False,
+        index=True,
+        comment="所属知识库 ID",
+    )
+    user_id: Mapped[str] = mapped_column(
+        String(32),
+        ForeignKey("users.id"),
+        nullable=False,
+        index=True,
+        comment="所有者用户 ID (冗余，便于隔离校验)",
+    )
+    category_id: Mapped[str | None] = mapped_column(
+        String(32),
+        ForeignKey("document_categories.id"),
+        nullable=True,
+        comment="分类 ID",
+    )
+    original_filename: Mapped[str] = mapped_column(
+        String(512),
+        nullable=False,
+        comment="原始文件名 (仅展示)",
+    )
+    storage_path: Mapped[str] = mapped_column(
+        String(1024),
+        nullable=False,
+        comment="相对于 data/ 的物理存储路径",
+    )
+    markdown_path: Mapped[str | None] = mapped_column(
+        String(1024),
+        nullable=True,
+        comment="相对于 data/ 的 Markdown 文件路径",
+    )
+    file_size: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        comment="文件大小 (字节)",
+    )
+    mime_type: Mapped[str] = mapped_column(
+        String(127),
+        nullable=False,
+        comment="MIME 类型",
+    )
+    file_ext: Mapped[str] = mapped_column(
+        String(16),
+        nullable=False,
+        comment="文件扩展名 (.docx/.pdf)",
+    )
+    sha256: Mapped[str] = mapped_column(
+        String(64),
+        nullable=False,
+        comment="文件 SHA-256 哈希",
+    )
+    doc_token_hash: Mapped[str] = mapped_column(
+        String(64),
+        unique=True,
+        nullable=True,
+        index=True,
+        comment="SHA-256(document_token) 十六进制",
+    )
+    doc_token_encrypted: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="Fernet 加密的 document_token 原文",
+    )
+    doc_token_hint: Mapped[str | None] = mapped_column(
+        String(16),
+        nullable=True,
+        comment="document_token 末 8 位明文",
+    )
+    title: Mapped[str | None] = mapped_column(
+        String(512),
+        nullable=True,
+        comment="文档标题 (用户可改)",
+    )
+    description: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="文档描述 (用户可改)",
+    )
+    keywords: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="关键词 (逗号分隔)",
+    )
+    content_summary: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="抽取式摘要 (~200字)",
+    )
+    status: Mapped[str] = mapped_column(
+        String(16),
+        default="PENDING",
+        nullable=False,
+        comment="状态 (PENDING/PROCESSING/READY/FAILED/DELETED)",
+    )
+    error_code: Mapped[str | None] = mapped_column(
+        String(64),
+        nullable=True,
+        comment="错误码 (如 SCANNED_PDF_NO_TEXT_LAYER)",
+    )
+
+    # 关系
+    knowledge_base = relationship("KnowledgeBase", back_populates="documents", lazy="selectin")
+    user = relationship("User", back_populates="documents", lazy="selectin")
+    category = relationship("DocumentCategory", back_populates="documents", lazy="selectin")
+    access_logs = relationship("AccessLog", back_populates="document", lazy="selectin")
+
+    def __repr__(self) -> str:
+        return f"<Document {self.original_filename!r} (status={self.status!r})>"
diff --git a/backend/app/models/document_category.py b/backend/app/models/document_category.py
new file mode 100644
index 0000000..ee450ae
--- /dev/null
+++ b/backend/app/models/document_category.py
@@ -0,0 +1,41 @@
+"""DocumentCategory 文档分类模型。"""
+
+from sqlalchemy import ForeignKey, Integer, String
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class DocumentCategory(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """文档分类模型。
+
+    技术审查 §2.2：document_categories 表。
+    """
+
+    __tablename__ = "document_categories"
+
+    knowledge_base_id: Mapped[str] = mapped_column(
+        String(32),
+        ForeignKey("knowledge_bases.id"),
+        nullable=False,
+        index=True,
+        comment="所属知识库 ID",
+    )
+    name: Mapped[str] = mapped_column(
+        String(255),
+        nullable=False,
+        comment="分类名称",
+    )
+    sort_order: Mapped[int] = mapped_column(
+        Integer,
+        default=0,
+        nullable=False,
+        comment="排序序号",
+    )
+
+    # 关系
+    knowledge_base = relationship("KnowledgeBase", back_populates="categories", lazy="selectin")
+    documents = relationship("Document", back_populates="category", lazy="selectin")
+
+    def __repr__(self) -> str:
+        return f"<DocumentCategory {self.name!r} (kb={self.knowledge_base_id!r})>"
diff --git a/backend/app/models/knowledge_base.py b/backend/app/models/knowledge_base.py
new file mode 100644
index 0000000..9f14a2b
--- /dev/null
+++ b/backend/app/models/knowledge_base.py
@@ -0,0 +1,65 @@
+"""KnowledgeBase 知识库模型。"""
+
+from sqlalchemy import Boolean, ForeignKey, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class KnowledgeBase(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """知识库模型。
+
+    技术审查 §2.2：knowledge_bases 表。
+    """
+
+    __tablename__ = "knowledge_bases"
+
+    user_id: Mapped[str] = mapped_column(
+        String(32),
+        ForeignKey("users.id"),
+        nullable=False,
+        index=True,
+        comment="所有者用户 ID",
+    )
+    name: Mapped[str] = mapped_column(
+        String(255),
+        nullable=False,
+        comment="知识库名称",
+    )
+    description: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="知识库描述",
+    )
+    enabled: Mapped[bool] = mapped_column(
+        Boolean,
+        default=True,
+        nullable=False,
+        comment="是否启用",
+    )
+    token_hash: Mapped[str] = mapped_column(
+        String(64),
+        unique=True,
+        nullable=False,
+        index=True,
+        comment="SHA-256(secret_token) 十六进制",
+    )
+    token_encrypted: Mapped[str | None] = mapped_column(
+        Text,
+        nullable=True,
+        comment="Fernet 加密的 token 原文",
+    )
+    token_hint: Mapped[str | None] = mapped_column(
+        String(16),
+        nullable=True,
+        comment="token 末 8 位明文，供后台识别",
+    )
+
+    # 关系
+    user = relationship("User", back_populates="knowledge_bases", lazy="selectin")
+    documents = relationship("Document", back_populates="knowledge_base", lazy="selectin")
+    categories = relationship("DocumentCategory", back_populates="knowledge_base", lazy="selectin")
+    access_logs = relationship("AccessLog", back_populates="knowledge_base", lazy="selectin")
+
+    def __repr__(self) -> str:
+        return f"<KnowledgeBase {self.name!r} (user={self.user_id!r})>"
diff --git a/backend/app/models/plan.py b/backend/app/models/plan.py
new file mode 100644
index 0000000..5991807
--- /dev/null
+++ b/backend/app/models/plan.py
@@ -0,0 +1,49 @@
+"""Plan 套餐模型。"""
+
+from sqlalchemy import Boolean, Integer, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class Plan(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """用户套餐（免费/基础/专业等）。
+
+    技术审查 §2.2：plans 表。
+    """
+
+    __tablename__ = "plans"
+
+    code: Mapped[str] = mapped_column(
+        String(32),
+        unique=True,
+        nullable=False,
+        comment="套餐代码 (free/basic/pro)",
+    )
+    name: Mapped[str] = mapped_column(
+        String(64),
+        nullable=False,
+        comment="套餐名称",
+    )
+    storage_quota: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        comment="存储配额 (字节)",
+    )
+    max_file_size: Mapped[int] = mapped_column(
+        Integer,
+        nullable=False,
+        comment="单文件大小上限 (字节)",
+    )
+    is_active: Mapped[bool] = mapped_column(
+        Boolean,
+        default=True,
+        nullable=False,
+        comment="是否可用",
+    )
+
+    # 关系
+    users = relationship("User", back_populates="plan", lazy="selectin")
+
+    def __repr__(self) -> str:
+        return f"<Plan {self.code!r} ({self.name!r})>"
diff --git a/backend/app/models/user.py b/backend/app/models/user.py
new file mode 100644
index 0000000..4e8ab66
--- /dev/null
+++ b/backend/app/models/user.py
@@ -0,0 +1,61 @@
+"""User 用户模型。"""
+
+from sqlalchemy import ForeignKey, Integer, String, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+
+
+class User(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+    """用户模型。
+
+    技术审查 §2.2：users 表。
+    """
+
+    __tablename__ = "users"
+
+    username: Mapped[str] = mapped_column(
+        String(64),
+        unique=True,
+        nullable=False,
+        index=True,
+        comment="用户名",
+    )
+    email: Mapped[str] = mapped_column(
+        String(255),
+        unique=True,
+        nullable=False,
+        index=True,
+        comment="邮箱",
+    )
+    password_hash: Mapped[str] = mapped_column(
+        String(255),
+        nullable=False,
+        comment="Argon2id 密码哈希",
+    )
+    status: Mapped[str] = mapped_column(
+        String(16),
+        default="active",
+        nullable=False,
+        comment="状态 (active/disabled)",
+    )
+    plan_id: Mapped[str] = mapped_column(
+        String(32),
+        ForeignKey("plans.id"),
+        nullable=False,
+        comment="套餐 ID",
+    )
+    storage_used: Mapped[int] = mapped_column(
+        Integer,
+        default=0,
+        nullable=False,
+        comment="已用存储 (字节)",
+    )
+
+    # 关系
+    plan = relationship("Plan", back_populates="users", lazy="selectin")
+    knowledge_bases = relationship("KnowledgeBase", back_populates="user", lazy="selectin")
+    documents = relationship("Document", back_populates="user", lazy="selectin")
+
+    def __repr__(self) -> str:
+        return f"<User {self.username!r}>"
diff --git a/docs/requirements.md b/docs/requirements.md
index 07eaa16..46fbf02 100644
--- a/docs/requirements.md
+++ b/docs/requirements.md
@@ -11,7 +11,7 @@
 | Phase | 内容 | 状态 |
 |---|---|---|
 | 1 | 项目初始化 | ✅ 完成 |
-| 2 | 数据库和 Alembic | ⬜ |
+| 2 | 数据库和 Alembic | ✅ 完成 |
 | 3 | 用户注册登录 | ⬜ |
 | 4 | 知识库 CRUD | ⬜ |
 | 5 | 本地 StorageService | ✅ 完成（提前实现于 Phase 1） |
@@ -30,7 +30,7 @@
 
 ---
 
-## 一、产品核心目标 ⬜（产品定位，随各 Phase 逐步实现）
+## 一、产品核心目标 🔄 产品定位，随各 Phase 逐步实现（Phase 1-2 已完成）
 
 这个产品不是普通网盘。
 
@@ -66,7 +66,7 @@
 
 生产环境：Ubuntu、Docker、Docker Compose。
 
-## 四、第一版系统架构 ⬜（架构已定，实现随 Phase 推进）
+## 四、第一版系统架构 🔄 架构已定 + 数据库层 ✅ 实现随 Phase 推进
 
 采用：
 
@@ -89,7 +89,7 @@ SQLite 保存元数据。文件系统保存：Word、PDF、Markdown。
 
 禁止把原始 PDF、Word 二进制内容存进 SQLite。SQLite 只保存：用户信息、知识库信息、文档元数据、文件路径、Markdown 路径、文档状态、Token hash、时间、分类、描述、关键词、搜索相关数据。原始文件与 Markdown 均存服务器文件系统。
 
-## 六、数据库访问必须抽象 ⬜ Phase 2（Phase 1 已建 db.py 引擎层）
+## 六、数据库访问必须抽象 ✅ 完成（Phase 2：ORM 模型 + Alembic 迁移已落地）
 
 不能在业务代码中到处直接调用 sqlite3。必须使用 SQLAlchemy，并通过 Repository / Service 分层（UserRepository、KnowledgeBaseRepository、DocumentRepository）。业务逻辑不能依赖 SQLite 具体实现。数据库配置通过 `DATABASE_URL`（如 `sqlite:///./data/app.db`），未来可以切换 PostgreSQL 而尽量不修改业务层。
 
@@ -123,7 +123,7 @@ SQLite 保存元数据。文件系统保存：Word、PDF、Markdown。
 
 User：id、username、email、password_hash、status、storage_quota、storage_used、created_at、updated_at。密码必须安全哈希，优先 Argon2id；禁止明文密码、MD5、SHA1。
 
-## 十四、存储限制 🔄 部分完成（config.py 已有配额配置；Plan 模型 Phase 2 落地）
+## 十四、存储限制 ✅ 完成（config.py 配额配置 + Plan 模型已落地 Phase 2）
 
 默认免费用户 100MB，单文件 20MB。不要硬编码，放到配置或 Plan 模型。第一版 Free Plan（100MB / 20MB-per-file），未来 Basic 1GB、Pro 5GB、Enterprise 自定义。第一版不实现支付。
 
@@ -192,7 +192,7 @@ data/
 
 第一版不强制 OCR。如果 PDF 没有文本层，提示"该 PDF 可能是扫描件，当前版本暂不支持 OCR。"未来预留 OCRProcessor（PaddleOCR 等），第一版不加入 OCR。
 
-## 二十六、文档状态 ⬜ Phase 2（状态模型）/ Phase 7（流转）
+## 二十六、文档状态 🔄 状态模型 ✅ 完成（Phase 2）；流转 ⬜ Phase 7
 
 PENDING、PROCESSING、READY、FAILED、DELETED。虽然第一版同步处理，但状态模型必须保留，未来异步任务可以直接使用。
 
@@ -248,11 +248,11 @@ Vue 3、TypeScript、Vite、Element Plus、Pinia。UI 要求：现代、简洁
 
 第一版不需要 Redis，可以使用内存限流（如单 IP 每分钟一定次数），保护公共 AI URL。注意内存限流只适合单实例 MVP，代码中抽象 RateLimiter，未来可实现 RedisRateLimiter。（已实现 `core/rate_limit.py`：内存 TokenBucket + 配置化阈值。）
 
-## 四十、访问日志 ⬜ Phase 2（模型）/ Phase 10（记录）
+## 四十、访问日志 🔄 模型 ✅ 完成（Phase 2）；记录 ⬜ Phase 10
 
 第一版可简单记录：knowledge_base_id、document_id、timestamp、user_agent、请求类型。不要默认长期保存完整 IP。后台可显示访问次数、最近访问时间。不要声称可以准确判断访问者是不是 AI，使用"外部访问"而不是"AI 访问"。
 
-## 四十一、数据库模型 ⬜ Phase 2
+## 四十一、数据库模型 ✅ 完成（Phase 2：User/Plan/KnowledgeBase/DocumentCategory/Document/AccessLog 全部落地）
 
 至少：User、Plan、KnowledgeBase、Document、DocumentCategory、AccessLog。
 
@@ -260,7 +260,7 @@ Document：id、knowledge_base_id、user_id、document_token_hash、original_fil
 
 KnowledgeBase：id、user_id、name、description、secret_token_hash、enabled、created_at、updated_at。
 
-## 四十二、数据库迁移 ⬜ Phase 2
+## 四十二、数据库迁移 ✅ 完成（Phase 2：Alembic 初始化 + 初始迁移 initial_schema 已应用）
 
 使用 Alembic，即使 SQLite 也必须使用迁移。不要手工修改生产数据库。README 提供：初始化迁移、升级迁移、回滚迁移。