Use live MQTT readiness for server health checks

internal/web/server.go

@@ -40,6 +40,7 @@ type Snapshot struct {
 	Compatibility []Compatibility        `json:"compatibility"`
 }
 type Viewer struct {
+	ready    func() bool
 	cfg      config.Web
 	store    *events.Store
 	shutdown time.Duration
@@ -60,10 +61,25 @@ func New(cfg config.Web, store *events.Store, shutdown time.Duration) *Viewer {
 	mux.Handle("GET /assets/", http.StripPrefix("/assets/", http.FileServer(http.FS(sub))))
 	mux.HandleFunc("GET /api/snapshot", v.snapshot)
 	mux.HandleFunc("GET /api/events", v.stream)
+	mux.HandleFunc("GET /healthz", v.health)
 	v.handler = v.guard(mux)
 	v.server = &http.Server{Handler: v.handler, ReadHeaderTimeout: 3 * time.Second, IdleTimeout: 20 * time.Second, MaxHeaderBytes: 8 << 10}
 	return v
 }
+
+// SetReadiness must be called before Start. Readiness reflects live MQTT state,
+// independent of connection events rolling out of the bounded viewer history.
+func (v *Viewer) SetReadiness(ready func() bool) { v.ready = ready }
+
+func (v *Viewer) health(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Cache-Control", "no-store")
+	if v.ready == nil || !v.ready() {
+		http.Error(w, "MQTT not ready", http.StatusServiceUnavailable)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_, _ = w.Write([]byte(`{"status":"ready"}`))
+}
 func (v *Viewer) Address() string {
 	if v.listener == nil {
 		return v.cfg.Address