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/b2-credentials-externalsecret.yaml

@@ -0,0 +1,25 @@
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: rancher-b2-creds
+  namespace: cattle-resources-system
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    name: doppler-hetznerterra
+    kind: ClusterSecretStore
+  target:
+    name: rancher-b2-creds
+    creationPolicy: Owner
+    template:
+      type: Opaque
+      data:
+        aws_access_key_id: "{{ .B2_ACCOUNT_ID }}"
+        aws_secret_access_key: "{{ .B2_APPLICATION_KEY }}"
+  data:
+    - secretKey: B2_ACCOUNT_ID
+      remoteRef:
+        key: B2_ACCOUNT_ID
+    - secretKey: B2_APPLICATION_KEY
+      remoteRef:
+        key: B2_APPLICATION_KEY