feat: per-service status checks with offline colouring

Adds optional live status checking per service (not just per node),
requested as a follow-up to issue #196.

Backend:
- New check_service / check_services: HTTP(S) GET for web services, TCP
  connect otherwise; UDP and port-less non-web services stay 'unknown'.
- New scheduler job 'service_checks', independent interval (default 300s),
  added/removed live via set_service_checks_enabled.
- Settings gain service_check_enabled + service_check_interval (>=30s),
  persisted to scan_config.json. New WS message type 'service_status'.

Frontend:
- Live per-service status overlay in canvasStore (not persisted, so it
  never round-trips through canvas save), fed by the WS message.
- DetailPanel + canvas node service rows: offline service turns red
  (#f85149), otherwise keeps its category colour.
- SettingsModal: toggle + interval input (default 300s / 5 min).

Off by default — no behaviour change until enabled.

ha-relevant: yes

backend/app/api/routes/status.py

@@ -54,6 +54,15 @@ async def broadcast_status(node_id: str, status: str, checked_at: str, response_
     }))
 
 
+async def broadcast_service_status(node_id: str, services: list[dict], checked_at: str) -> None:
+    await _broadcast(json.dumps({
+        "type": "service_status",
+        "node_id": node_id,
+        "services": services,
+        "checked_at": checked_at,
+    }))
+
+
 async def broadcast_scan_update(run_id: str, devices_found: int) -> None:
     await _broadcast(json.dumps({
         "type": "scan_device_found",