Refactor(backend): improve formatting in services, repositories & tests

- Consistently format multi-line function headers, exception handling, and repository method calls for readability.
- Reorganize misplaced imports across modules (e.g., services & tests) into proper sorted order.
- Adjust indentation, line breaks, and spacing inconsistencies in tests and migration files.
- Cleanup unnecessary trailing newlines and reorganize `__all__` declarations for consistency.

backend/app/repositories/oauth_client.py

@@ -22,7 +22,9 @@ class EmptySchema(BaseModel):
     """Placeholder schema for repository operations that don't need update schemas."""
 
 
-class OAuthClientRepository(BaseRepository[OAuthClient, OAuthClientCreate, EmptySchema]):
+class OAuthClientRepository(
+    BaseRepository[OAuthClient, OAuthClientCreate, EmptySchema]
+):
     """Repository for OAuth clients (provider mode)."""
 
     async def get_by_client_id(