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/tests/services/test_user_service.py

@@ -4,13 +4,12 @@
 import uuid
 
 import pytest
-import pytest_asyncio
 from sqlalchemy import select
 
 from app.core.exceptions import NotFoundError
 from app.models.user import User
 from app.schemas.users import UserCreate, UserUpdate
-from app.services.user_service import UserService, user_service
+from app.services.user_service import user_service
 
 
 class TestGetUser: