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

backend/app/schemas/link.py

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

backend/app/services/device.py

@@ -119,7 +119,6 @@ def _build_device_response(device: Device) -> DeviceResponse:
 
 def _device_with_relations():
     """Return a select() for Device with tags and groups eagerly loaded."""
-    from app.models.site import Site  # noqa: F811
 
     return select(Device).options(
         selectinload(Device.tag_assignments).selectinload(DeviceTagAssignment.tag),

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,

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

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