mqtt setup panel: gate on enable toggle, reorder, move next to jellyfin

- new mqtt_enabled config + toggle at top of the section; subscriber
  only starts when the box is checked
- moved the whole MqttSection directly below the Jellyfin section so
  all jellyfin-adjacent config lives together
- rewrote the plugin setup list to match the actual form order and
  group it: 'Top of plugin page' (Server Url = jellyfin base URL),
  'Generic destination', 'MQTT settings', 'Template'
- fields the user picks from a dropdown or toggles (Status,
  Notification Type, Item Type, Use TLS, Use Credentials, QoS) now
  render a 'select' hint instead of a broken Copy button

server/db/index.ts

@@ -22,6 +22,7 @@ const ENV_MAP: Record<string, string> = {
 	sonarr_api_key: "SONARR_API_KEY",
 	sonarr_enabled: "SONARR_ENABLED",
 	audio_languages: "AUDIO_LANGUAGES",
+	mqtt_enabled: "MQTT_ENABLED",
 	mqtt_url: "MQTT_URL",
 	mqtt_topic: "MQTT_TOPIC",
 	mqtt_username: "MQTT_USERNAME",