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/rancher-backup/backup-recurring.yaml

@@ -0,0 +1,17 @@
+apiVersion: resources.cattle.io/v1
+kind: Backup
+metadata:
+  name: rancher-b2-recurring
+  namespace: cattle-resources-system
+spec:
+  resourceSetName: rancher-resource-set-full
+  storageLocation:
+    s3:
+      credentialSecretName: rancher-b2-creds
+      credentialSecretNamespace: cattle-resources-system
+      bucketName: HetznerTerra
+      folder: rancher-backups
+      endpoint: s3.us-east-005.backblazeb2.com
+      region: us-east-005
+  schedule: "0 3 * * *"
+  retentionCount: 7