ops: glance borg-backup-widget via prometheus + synthwave/matrix presets

- glance zusaetzlich in monitoring_net (nur lesende Prometheus-Query, kein neuer Listener)
- Borg-Widget: Backup-Alter aus homelab_borg_last_completed_timestamp_seconds, Status aus homelab_borg_last_success
- Theme-Presets synthwave und matrix

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

ops/glance/docker-compose.yml

@@ -20,6 +20,8 @@ services:
     networks:
       - frontend_net
       - glance_socket_net
+      # monitoring_net nur lesend fuer Prometheus-Query des Borg-Backup-Widgets
+      - monitoring_net
     depends_on:
       - glance-docker-socket-proxy
     labels:
@@ -56,6 +58,8 @@ services:
 networks:
   frontend_net:
     external: true
+  monitoring_net:
+    external: true
   glance_socket_net:
     name: glance_socket_net
     internal: true