Fix byte order of track_id published over MQTT

The 'mper' handler cast the raw big-endian persistent-id bytes directly
to a host-native uint64_t, which reverses the value on little-endian
hosts (e.g. Raspberry Pi). metadata_hub.c already reconstructs the same
field correctly by applying ntohl() to each 32-bit half; mqtt.c now does
the same, so the published track_id matches the real dmap.persistentid
and can be used directly with the queue_next command.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mqtt.c

@@ -347,7 +347,9 @@ void mqtt_process_metadata(uint32_t type, uint32_t code, char *data, uint32_t le
         mqtt_publish("title", data, length);
         break;
       case 'mper':
-        trackid = *(uint64_t *)(data);
+        trackid = ntohl(*(uint32_t *)data);
+        trackid = trackid << 32;
+        trackid += ntohl(*(uint32_t *)(data + sizeof(uint32_t)));
         r = snprintf(trackidstring, sizeof(trackidstring), "%" PRIX64 "", trackid);
         mqtt_publish("track_id", trackidstring, r);
       }