Give demo app a real Longhorn-backed PVC

apps/demo/pvc.yaml

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+# A hands-on check that Longhorn actually works (see docs/06-longhorn.md).
+# No storageClassName is set, so Kubernetes uses whichever StorageClass is
+# marked default — Longhorn's own chart marks itself default, so this PVC
+# gets real, replicated storage on the two Proxmox workers without naming
+# Longhorn here explicitly.
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: demo-data
+  namespace: demo
+spec:
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 2Gi