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.yaml

@@ -10,7 +10,6 @@ resources:
   - kustomization-flux-ui.yaml
   - kustomization-observability.yaml
   - kustomization-observability-content.yaml
-  - kustomization-cnpg-operator.yaml
-  - kustomization-cnpg.yaml
   - kustomization-rancher.yaml
   - kustomization-rancher-config.yaml
+  - kustomization-rancher-backup.yaml