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/routers/sectors.py

@@ -11,7 +11,6 @@ RBAC:
 """
 
 import uuid
-from typing import Optional
 
 from fastapi import APIRouter, Depends, status
 from pydantic import BaseModel