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_account.py

@@ -23,7 +23,9 @@ class EmptySchema(BaseModel):
     """Placeholder schema for repository operations that don't need update schemas."""
 
 
-class OAuthAccountRepository(BaseRepository[OAuthAccount, OAuthAccountCreate, EmptySchema]):
+class OAuthAccountRepository(
+    BaseRepository[OAuthAccount, OAuthAccountCreate, EmptySchema]
+):
     """Repository for OAuth account links."""
 
     async def get_by_provider_id(