fix: ruff findings

tests/test_day_aggregates.py

@@ -7,9 +7,8 @@ From spec §5.4, §3.3, ST-4, ST-5:
   whose classification is known
 - No absent hour ever interpolated/estimated/fabricated (FL-3)
 """
-import sqlite3
 import sys
-from datetime import datetime, timezone, timedelta
+from datetime import datetime, timezone
 from pathlib import Path
 
 import pytest
@@ -18,8 +17,8 @@ sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
 
 from fenris.store import init_store
 from fenris.monitoring_periods import ensure_period_open, close_period
-from fenris.day_aggregate import derive_day, derive_all_days, DayAggregate
-from fenris.hour_classify import HourSplit, ACTIVE_THRESHOLD_BYTES
+from fenris.day_aggregate import derive_day, derive_all_days
+from fenris.hour_classify import HourSplit
 
 
 @pytest.fixture