feat(agent): SendDisplayState im MQTT-Publisher + Interface

player/agent/internal/app/app.go

@@ -91,6 +91,7 @@ type statusSender interface {
 
 type mqttSender interface {
 	SendHeartbeat(status, connectivity string, ts time.Time) error
+	SendDisplayState(screenSlug, state string) error
 	Close()
 }
 

player/agent/internal/mqttheartbeat/heartbeat.go

@@ -78,6 +78,26 @@ func (p *Publisher) SendHeartbeat(status, connectivity string, ts time.Time) err
 	return token.Error()
 }
 
+// SendDisplayState publiziert den aktuellen Display-Zustand auf dem display-state-Topic.
+// QoS 0, nicht retained — Informationszweck/Monitoring.
+func (p *Publisher) SendDisplayState(screenSlug, state string) error {
+	type dsPayload struct {
+		DisplayState string `json:"display_state"`
+		Timestamp    string `json:"ts"`
+	}
+	data, err := json.Marshal(dsPayload{
+		DisplayState: state,
+		Timestamp:    time.Now().UTC().Format(time.RFC3339),
+	})
+	if err != nil {
+		return err
+	}
+	topic := "signage/screen/" + screenSlug + "/display-state"
+	token := p.client.Publish(topic, 0, false, data)
+	token.WaitTimeout(3 * time.Second)
+	return token.Error()
+}
+
 // Close disconnects from the broker gracefully.
 func (p *Publisher) Close() {
 	p.client.Disconnect(250)