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/repositories/test_oauth.py

@@ -63,7 +63,8 @@ class TestOAuthAccountCRUD:
 
             # SQLite returns different error message than PostgreSQL
             with pytest.raises(
-                DuplicateEntryError, match="(already linked|UNIQUE constraint failed|Failed to create)"
+                DuplicateEntryError,
+                match="(already linked|UNIQUE constraint failed|Failed to create)",
             ):
                 await oauth_account.create_account(session, obj_in=account_data2)