prepare fastapi root layout for server deployment

app/models/mixins.py

@@ -0,0 +1,15 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime
+from sqlalchemy.orm import Mapped, mapped_column
+
+
+class TimestampMixin:
+    """时间字段：统一提供创建时间和更新时间。"""
+
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime,
+        default=datetime.utcnow,
+        onupdate=datetime.utcnow,
+    )