第二步

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)",
+    )