fix: ruff findings

tests/test_cross_day_duplication.py

@@ -6,7 +6,7 @@ shared boundary evidence, not duplicated into both days.
 Seam: derive._add_unattributed_bytes() → day_aggregates.unattributed_bytes_*
 """
 import sqlite3
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timezone
 from pathlib import Path
 
 import pytest
@@ -14,8 +14,6 @@ import sys
 sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
 
 from fenris.collector import run_collection
-from fenris.store import init_store
-from fenris.monitoring_periods import ensure_period_open
 
 
 def _make_smartctl(duw: int, dur: int):