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/site_service.py

@@ -8,7 +8,7 @@ import uuid
 
 import structlog
 from fastapi import HTTPException, status
-from sqlalchemy import func, select, text, update
+from sqlalchemy import func, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.device import Device