Step 3：Study / Site / StudyMember + 项目权限

backend/app/db/base.py

@@ -1,5 +1,7 @@
-from sqlalchemy.orm import DeclarativeBase
+from app.db.base_class import Base
 
-
-class Base(DeclarativeBase):
-    pass
+# Import models to ensure metadata is loaded for create_all
+from app.models.user import User  # noqa: F401
+from app.models.study import Study  # noqa: F401
+from app.models.site import Site  # noqa: F401
+from app.models.study_member import StudyMember  # noqa: F401

backend/app/db/base_class.py

@@ -0,0 +1,5 @@
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    pass