hub: actually send start/stop commands over MQTT

The /cameras/{id}/start and /stop handlers only wrote a recording_events
row — they never published the command, so the camera never recorded.
Add Subscriber.PublishCommand (publishes {"command":...} to
remoterig/cameras/<id>/command, which the XIAO forwards to the ESP-01S),
thread a CommandPublisher into the recording handlers, and wire mqttSub in
via apiRouter. Tests pass nil (publish skipped).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/mqtt/subscriber.go

@@ -143,6 +143,18 @@ type statusPayload struct {
 	UptimeSec         *int    `json:"uptime_sec"`
 }
 
+// PublishCommand sends a command (e.g. "start_recording") to a camera's
+// command topic, which its ESP32 bridge subscribes to and forwards over UART.
+func (s *Subscriber) PublishCommand(cameraID, command string) error {
+	topic := "remoterig/cameras/" + cameraID + "/command"
+	payload, _ := json.Marshal(map[string]string{"command": command})
+	tok := s.client.Publish(topic, 2, false, payload)
+	if !tok.WaitTimeout(3 * time.Second) {
+		return fmt.Errorf("publish to %s timed out", topic)
+	}
+	return tok.Error()
+}
+
 func (s *Subscriber) handleStatus(cameraID string, payload []byte) {
 	var sp statusPayload
 	if err := json.Unmarshal(payload, &sp); err != nil {