fix(lint): remove unused imports and extraneous f-string prefix

Ruff auto-fix: unused Optional imports in sectors router and link
schemas, unused Site import in device service, unused datetime
imports in trend detector, unused text import in site service,
and f-string without placeholders in signal history service.

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

backend/app/services/signal_history_service.py

@@ -41,7 +41,7 @@ async def get_signal_history(
     bucket_interval, lookback = RANGE_CONFIG.get(range, RANGE_CONFIG["7d"])
 
     result = await db.execute(
-        text(f"""
+        text("""
             SELECT
                 time_bucket(:bucket_interval, wr.time) AS bucket,
                 avg(wr.signal_strength)::int AS signal_avg,