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/session.py

@@ -10,7 +10,7 @@ from sqlalchemy import and_, delete, func, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import joinedload
 
-from app.core.repository_exceptions import InvalidInputError, IntegrityConstraintError
+from app.core.repository_exceptions import IntegrityConstraintError, InvalidInputError
 from app.models.user_session import UserSession
 from app.repositories.base import BaseRepository
 from app.schemas.sessions import SessionCreate, SessionUpdate