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/tests/test_translate_scheduler_execution.py

@@ -9,10 +9,10 @@
 # @TEST_INVARIANT: trigger_type_dispatch -> VERIFIED_BY: [test_new_key_only_mode, test_baseline_expired_fallback, test_full_mode_default]
 import sys
 from pathlib import Path
-sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
-from unittest.mock import MagicMock, patch
-from datetime import datetime, timezone, timedelta
 
+sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
+from datetime import UTC, datetime, timedelta
+from unittest.mock import MagicMock, patch
 
 # -- Helpers -----------------------------------------------------------------
 
@@ -23,7 +23,7 @@ def _make_most_recent_run(created_at_delta_days=60):
     run.job_id = "job-1"
     run.status = "COMPLETED"
     run.insert_status = "succeeded"
-    run.created_at = datetime.now(timezone.utc) - timedelta(days=created_at_delta_days)
+    run.created_at = datetime.now(UTC) - timedelta(days=created_at_delta_days)
     return run