refactor: Replace CNPG external DB with rancher-backup operator

Rancher 2.x uses embedded etcd, not an external PostgreSQL database.
The CATTLE_DB_CATTLE_* env vars are Rancher v1 only and were ignored.

- Remove all CNPG (CloudNativePG) cluster, operator, and related configs
- Remove external DB env vars from Rancher HelmRelease
- Remove rancher-db-password ExternalSecret
- Add rancher-backup operator HelmRelease (v106.0.2+up8.1.0)
- Add B2 credentials ExternalSecret for backup storage
- Add recurring Backup CR (daily at 03:00, 7 day retention)
- Add commented-out Restore CR for rebuild recovery
- Update Flux dependency graph accordingly

infrastructure/addons/kustomization-rancher-backup.yaml

@@ -0,0 +1,18 @@
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: addon-rancher-backup
+  namespace: flux-system
+spec:
+  interval: 10m
+  prune: true
+  sourceRef:
+    kind: GitRepository
+    name: platform
+  path: ./infrastructure/addons/rancher-backup
+  wait: true
+  timeout: 10m
+  suspend: false
+  dependsOn:
+    - name: addon-external-secrets
+    - name: addon-rancher