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/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),