feat: Alembic setup, User and AuditLog models with migration

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/models/audit.py

@@ -0,0 +1,20 @@
+"""审计日志 ORM 模型。"""
+import uuid
+from datetime import datetime
+from sqlalchemy import String, DateTime, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class AuditLog(Base):
+    __tablename__ = "audit_logs"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    user_id: Mapped[str] = mapped_column(String(36), nullable=False, index=True)
+    username: Mapped[str] = mapped_column(String(50), nullable=False)
+    action: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    target: Mapped[str] = mapped_column(String(255), nullable=False)
+    detail: Mapped[str | None] = mapped_column(Text, nullable=True)
+    ip_address: Mapped[str | None] = mapped_column(String(45), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)