fix: ruff findings

tests/test_monitoring_periods.py

@@ -6,9 +6,8 @@ From spec §5.2, §8.6, §9.8:
 - Powered-off time stays inside a period; disabled time does not
 - End causes: user_disabled, migrated, unknown_gap
 """
-import sqlite3
 import sys
-from datetime import datetime, timezone, timedelta
+from datetime import datetime, timezone
 from pathlib import Path
 
 import pytest