Add missing n8n-main-persistence PVC on VPS

n8n's chart uses persistence.type: existing, which expects this PVC to
already exist - nothing ever created it since n8n started at replicas: 0
and is only now being scaled up to 1 (confirmed live: pod stuck Pending,
'persistentvolumeclaim n8n-main-persistence not found').

infrastructure/vps-standby/n8n/manifests/pvc.yaml

@@ -0,0 +1,17 @@
+---
+# n8n's chart uses persistence.type: existing (values.yaml) - it expects
+# this PVC to already exist rather than creating one itself. At home this
+# PVC predates the chart deploy; on the VPS nothing ever created it since
+# n8n started at replicas: 0 - found live when scaling up to 1.
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: n8n-main-persistence
+  namespace: n8n
+spec:
+  accessModes:
+    - ReadWriteOnce
+  storageClassName: local-path
+  resources:
+    requests:
+      storage: 10Gi