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/helmrelease-rancher-backup.yaml

@@ -0,0 +1,28 @@
+apiVersion: helm.toolkit.fluxcd.io/v2
+kind: HelmRelease
+metadata:
+  name: rancher-backup
+  namespace: flux-system
+spec:
+  interval: 10m
+  targetNamespace: cattle-resources-system
+  chart:
+    spec:
+      chart: rancher-backup
+      version: "106.0.2+up8.1.0"
+      sourceRef:
+        kind: HelmRepository
+        name: rancher-charts
+        namespace: flux-system
+  install:
+    createNamespace: true
+    remediation:
+      retries: 3
+  upgrade:
+    remediation:
+      retries: 3
+  values:
+    image:
+      repository: rancher/backup-restore-operator
+    s3:
+      enabled: true