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_session_service.py

@@ -5,10 +5,9 @@ import uuid
 from datetime import UTC, datetime, timedelta
 
 import pytest
-import pytest_asyncio
 
 from app.schemas.sessions import SessionCreate
-from app.services.session_service import SessionService, session_service
+from app.services.session_service import session_service
 
 
 def _make_session_create(user_id, jti=None) -> SessionCreate: