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/schemas/link.py

@@ -2,7 +2,6 @@
 
 import uuid
 from datetime import datetime
-from typing import Optional
 
 from pydantic import BaseModel, ConfigDict