Add VPS warm-standby/backup site (Phase 0-1b)

Foundation for a DR/backup path using an always-on VPS as a second
ArgoCD-managed cluster, plus DB/backup standardization work that fell
out of it:

- vps-standby ArgoCD cluster destination + AppProject, MinIO backup
  receiver, VPS bootstrap script (k3s, Netbird, cert-manager)
- Dual-site DNS failover watcher + home-IP DDNS CronJob, Cloudflare
  token moved out of git into Vault+ExternalSecret
- Nextcloud migrated from ad-hoc MariaDB to CNPG + redis-operator
  (matches n8n/Authentik/GitLab's backup-native pattern)
- Authentik's CNPG manifests moved into the actual ArgoCD-synced
  manifests/ path (were present but never wired into the sync path)
- Vault raft-snapshot CronJob, CNPG barmanObjectStore backups
  (Authentik/n8n/Nextcloud), Nextcloud file-PVC restic sync - all
  targeting the new VPS MinIO receiver

See VPS Warm-Standby plan doc for full design rationale.

apps/n8n/manifests/cnpg-cluster.yaml

@@ -47,5 +47,25 @@ spec:
       database: n8n
       owner: n8n
 
+  # Backup to the VPS MinIO backup receiver (Phase 1b). Previously had no
+  # backup block at all. VPS_MINIO_ENDPOINT placeholder matches
+  # infrastructure/vault/manifests/raft-snapshot-cronjob.yaml - replace with
+  # the VPS's actual Netbird address once bootstrapped.
+  backup:
+    barmanObjectStore:
+      destinationPath: s3://cnpg-backups/pg-n8n
+      endpointURL: http://vps-minio.netbird.internal:30900
+      s3Credentials:
+        accessKeyId:
+          name: vps-minio-secret
+          key: accesskey
+        secretAccessKey:
+          name: vps-minio-secret
+          key: secretkey
+      wal:
+        compression: gzip
+        maxParallel: 2
+    retentionPolicy: "30d"
+
   monitoring:
     enablePodMonitor: true

apps/n8n/manifests/vps-minio-secret.yaml

@@ -0,0 +1,26 @@
+---
+# VPS MinIO credentials for CNPG's barmanObjectStore backup target.
+# Same Vault source as infrastructure/vault/manifests/raft-snapshot-cronjob.yaml
+# (secret/vps-minio-credentials) - populated once, manually, after VPS bootstrap.
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: vps-minio-credentials
+  namespace: n8n
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    kind: ClusterSecretStore
+    name: vault-backend
+  target:
+    name: vps-minio-secret
+    creationPolicy: Owner
+  data:
+    - secretKey: accesskey
+      remoteRef:
+        key: vps-minio-credentials
+        property: access-key
+    - secretKey: secretkey
+      remoteRef:
+        key: vps-minio-credentials
+        property: secret-key