adapt __init__.py files for services and repositories

Author: MoSchmidt

backend/app/repositories/__init__.py

@@ -0,0 +1,3 @@
+from .user_repository import UserRepository
+
+__all__ = ["UserRepository"]

backend/app/services/__init__.py

@@ -0,0 +1,4 @@
+from .auth_service import AuthService
+from .user_service import UserService
+
+__all__ = ["AuthService", "UserService"]