Add disk usage metrics to Sifaka NAS widget

Shows used and total space from /Volumes/backups mount

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
+            widget:
+              type: prometheusmetric
+              url: https://prometheus.ops.eblu.me
+              metrics:
+                - label: Used
+                  query: node_filesystem_size_bytes{mountpoint="/Volumes/backups"} - node_filesystem_avail_bytes{mountpoint="/Volumes/backups"}
+                  format:
+                    type: bytes
+                - label: Total
+                  query: node_filesystem_size_bytes{mountpoint="/Volumes/backups"}
+                  format:
+                    type: bytes
         - Borgmatic:
             href: https://grafana.ops.eblu.me/d/borgmatic
             icon: borgmatic