fix: resolve ruff lint and formatting issues

Remove unused timedelta import from test_wireless_api.py and
auto-format metrics.py to pass ruff format check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/tests/integration/test_wireless_api.py

@@ -9,7 +9,7 @@ All tests run against real PostgreSQL+TimescaleDB.
 """
 
 import uuid
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timezone
 
 import pytest
 from sqlalchemy import text