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/trend_detector.py

@@ -10,7 +10,6 @@ Uses AdminAsyncSessionLocal (bypasses RLS -- trend detection is system-level).
 """
 
 import asyncio
-from datetime import datetime, timezone
 
 import structlog
 from sqlalchemy import text