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/helmrelease-rancher.yaml

@@ -26,19 +26,6 @@ spec:
     tls: external
     replicas: 1
     extraEnv:
-      - name: CATTLE_DB_CATTLE_HOST
-        value: cnpg-cluster-rw.cnpg-cluster.svc
-      - name: CATTLE_DB_CATTLE_PORT
-        value: "5432"
-      - name: CATTLE_DB_CATTLE_DATABASE
-        value: postgres
-      - name: CATTLE_DB_CATTLE_USERNAME
-        value: postgres
-      - name: CATTLE_DB_CATTLE_PASSWORD
-        valueFrom:
-          secretKeyRef:
-            name: rancher-db-password
-            key: password
       - name: CATTLE_PROMETHEUS_METRICS
         value: "true"
     resources: