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/__tests__/client.test.ts

@@ -204,8 +204,8 @@ describe('api/client', () => {
   it('settingsApi get/save', () => {
     mod.settingsApi.get()
     expect(api.get).toHaveBeenCalledWith('/settings')
-    mod.settingsApi.save({ interval_seconds: 30 })
-    expect(api.post).toHaveBeenCalledWith('/settings', { interval_seconds: 30 })
+    mod.settingsApi.save({ interval_seconds: 30, service_check_enabled: true, service_check_interval: 600 })
+    expect(api.post).toHaveBeenCalledWith('/settings', { interval_seconds: 30, service_check_enabled: true, service_check_interval: 600 })
   })
 
   it('zigbeeApi.testConnection/importNetwork/importToPending', () => {