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/stores/__tests__/canvasStore.test.ts

@@ -31,9 +31,36 @@ describe('canvasStore', () => {
       past: [],
       future: [],
       clipboard: { nodes: [], edges: [] },
+      serviceStatuses: {},
     })
   })
 
+  it('setServiceStatuses stores live status keyed by node/port/protocol', () => {
+    const { setServiceStatuses } = useCanvasStore.getState()
+    setServiceStatuses('node-1', [
+      { port: 80, protocol: 'tcp', status: 'offline' },
+      { port: 443, protocol: 'tcp', status: 'online' },
+    ])
+    const { serviceStatuses } = useCanvasStore.getState()
+    expect(serviceStatuses['node-1:80/tcp']).toBe('offline')
+    expect(serviceStatuses['node-1:443/tcp']).toBe('online')
+  })
+
+  it('setServiceStatuses merges without dropping other nodes', () => {
+    const { setServiceStatuses } = useCanvasStore.getState()
+    setServiceStatuses('node-1', [{ port: 80, protocol: 'tcp', status: 'online' }])
+    setServiceStatuses('node-2', [{ port: 22, protocol: 'tcp', status: 'offline' }])
+    const { serviceStatuses } = useCanvasStore.getState()
+    expect(serviceStatuses['node-1:80/tcp']).toBe('online')
+    expect(serviceStatuses['node-2:22/tcp']).toBe('offline')
+  })
+
+  it('does not mark canvas unsaved on a service status update', () => {
+    useCanvasStore.setState({ hasUnsavedChanges: false })
+    useCanvasStore.getState().setServiceStatuses('n', [{ port: 80, protocol: 'tcp', status: 'offline' }])
+    expect(useCanvasStore.getState().hasUnsavedChanges).toBe(false)
+  })
+
   it('setEditingTextId sets and clears editing text id', () => {
     const { setEditingTextId } = useCanvasStore.getState()
     setEditingTextId('t1')