feat(events): broadcast state-invalidate on docker events so dashboard updates live (#768)

Dashboard and sidebar status indicators previously only refreshed on a
5-30 second polling cadence: a container restart, a degraded -> healthy
transition, or a stack update was invisible until the next tick.

Add a lightweight, non-persisted "state-invalidate" envelope on the
existing /ws/notifications WebSocket:

Backend
- NotificationService.broadcastEvent: sibling of dispatchAlert that
  pushes an arbitrary {type, ...} envelope to every subscriber WITHOUT
  writing to the alerts history (these are pure ephemeral signals).
- DockerEventService.handleEvent: emit the envelope for state-changing
  container actions (start/die/kill/destroy/create/restart/pause/
  unpause/health_status/rename/update). Carries node id, stack name
  (from the compose project label), container id, action, and
  timestamp.

Frontend
- EditorLayout's two notification WebSocket handlers (local plus
  per-remote-node) branch on type. On state-invalidate they re-emit a
  window CustomEvent and trigger a debounced (250ms) refreshStacks so
  a burst of events from compose recreating multiple services
  collapses to one refetch. The refresh callback is held in a ref so
  the long-lived WS effect never closes over a stale function.
- useDashboardData listens for the same window event and refetches
  /stats, /system/stats, and /stacks/statuses on every signal.
  Historical metrics stay on their 60s polling cadence (10-minute
  trend data, not a live indicator).

Tests
- Three new docker-event-service cases assert broadcastEvent fires on
  start and health_status events with the correct envelope shape, and
  does not fire on non-state actions like exec_create.
- Existing 28 cases updated with the broadcastEvent mock so the
  subscriber stub matches the new shape.

Polling stays as a safety net at the same intervals; the WS path is
the fast path. Multi-node fleets benefit on the local node today;
extending the remote forwarder to relay state-invalidate is a
recommended follow-up.

frontend/src/components/dashboard/useDashboardData.ts

@@ -164,6 +164,34 @@ export function useDashboardData(): DashboardData {
     return cleanup;
   }, [nodeId, fetchJson]);
 
+  // React to live `state-invalidate` signals from /ws/notifications: when a
+  // Docker container event fires (start/stop/die/restart/health), the layout
+  // re-broadcasts the envelope as a window CustomEvent. Refetch the cheap
+  // data (stats, system, statuses) immediately so the dashboard header and
+  // sidebar status update in well under a second instead of waiting for the
+  // next polling tick. Historical metrics are intentionally skipped — they
+  // are a 10-minute trend, not a live indicator.
+  useEffect(() => {
+    const currentNodeId = nodeId;
+    const onInvalidate = async () => {
+      if (nodeIdRef.current !== currentNodeId) return;
+      const [statsData, sysData, statusesData] = await Promise.all([
+        fetchJson<Stats>('/stats'),
+        fetchJson<SystemStats>('/system/stats'),
+        fetchJson<Record<string, StackStatusEntry>>('/stacks/statuses'),
+      ]);
+      if (nodeIdRef.current !== currentNodeId) return;
+      if (statsData) {
+        setStats(statsData);
+        setLastSyncAt(Date.now());
+      }
+      if (sysData) setSystemStats(sysData);
+      if (statusesData) setStackStatuses(statusesData);
+    };
+    window.addEventListener('sencho:state-invalidate', onInvalidate);
+    return () => window.removeEventListener('sencho:state-invalidate', onInvalidate);
+  }, [nodeId, fetchJson]);
+
   const stackCpuSeries = useMemo<Record<string, StackCpuSeries>>(() => {
     if (metrics.length === 0) return {};
     const grouped = new Map<string, MetricPoint[]>();