Fix n8n PVC: pin volumeName to existing PV

Prevents ArgoCD from unbinding the existing PV on sync.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

n8n/pvc.yaml

@@ -6,6 +6,7 @@ spec:
   accessModes:
   - ReadWriteOnce
   storageClassName: longhorn
+  volumeName: pvc-ebe4dd9c-8c9a-4485-91e0-9adf2b14b699
   resources:
     requests:
       storage: 5Gi