Add borgmatic backup status widget to Homepage

Displays time since last backup using prometheusmetric widget
querying Prometheus for borgmatic_last_archive_timestamp

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

argocd/manifests/homepage/values.yaml

@@ -44,6 +44,18 @@ config:
             href: https://nas.ops.eblu.me
             icon: synology
             description: NAS dashboard
+        - Borgmatic:
+            href: https://grafana.ops.eblu.me/d/borgmatic
+            icon: borgmatic
+            description: Backup system
+            widget:
+              type: prometheusmetric
+              url: https://prometheus.ops.eblu.me
+              metrics:
+                - label: Last backup
+                  query: time() - borgmatic_last_archive_timestamp
+                  format:
+                    type: duration
 
   # External bookmarks
   bookmarks: