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

frontend/src/api/client.ts

@@ -90,9 +90,15 @@ export const scanApi = {
   saveConfig: (data: { ranges: string[] }) => api.post('/scan/config', data),
 }
 
+export interface AppSettings {
+  interval_seconds: number
+  service_check_enabled: boolean
+  service_check_interval: number
+}
+
 export const settingsApi = {
-  get: () => api.get<{ interval_seconds: number }>('/settings'),
-  save: (data: { interval_seconds: number }) => api.post<{ interval_seconds: number }>('/settings', data),
+  get: () => api.get<AppSettings>('/settings'),
+  save: (data: AppSettings) => api.post<AppSettings>('/settings', data),
 }
 
 export const designsApi = {