chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/src/core/database.py

@@ -8,25 +8,27 @@
 #
 # @INVARIANT: A single engine instance is used for the entire application.
 
+import os
+from pathlib import Path
+
 from sqlalchemy import create_engine, inspect, text
 from sqlalchemy.orm import sessionmaker
-from ..models.mapping import Base
-from ..models.connection import ConnectionConfig
+
+from ..models import assistant as _assistant_models  # noqa: F401
+from ..models import auth as _auth_models  # noqa: F401
+from ..models import clean_release as _clean_release_models  # noqa: F401
+from ..models import config as _config_models  # noqa: F401
+from ..models import connection as _connection_models  # noqa: F401
+from ..models import dataset_review as _dataset_review_models  # noqa: F401
+from ..models import llm as _llm_models  # noqa: F401
+from ..models import profile as _profile_models  # noqa: F401
 
 # Import models to ensure they're registered with Base
 from ..models import task as _task_models  # noqa: F401
-from ..models import auth as _auth_models  # noqa: F401
-from ..models import config as _config_models  # noqa: F401
-from ..models import llm as _llm_models  # noqa: F401
-from ..models import assistant as _assistant_models  # noqa: F401
-from ..models import profile as _profile_models  # noqa: F401
-from ..models import clean_release as _clean_release_models  # noqa: F401
-from ..models import connection as _connection_models  # noqa: F401
-from ..models import dataset_review as _dataset_review_models  # noqa: F401
-from .logger import belief_scope, logger
+from ..models.connection import ConnectionConfig
+from ..models.mapping import Base
 from .auth.config import auth_config
-import os
-from pathlib import Path
+from .logger import belief_scope, logger
 
 # #region BASE_DIR [C:1] [TYPE Variable]
 # @BRIEF Base directory for the backend.