feat: 初始提交

backend/app/db/base.py

@@ -0,0 +1,9 @@
+from sqlalchemy.orm import DeclarativeBase, declared_attr
+
+
+class Base(DeclarativeBase):
+    """SQLAlchemy 基类，自动根据类名生成表名。"""
+
+    @declared_attr.directive
+    def __tablename__(cls) -> str:  # type: ignore[override]
+        return cls.__name__.lower()