feat(backup): wire Velero + CNPG Barman to the live MinIO S3 target

Author the GitOps side of off-cluster backups against the on-Proxmox MinIO
(http://192.168.1.9:9000, path-style; buckets velero + cnpg).

Velero (R-STATE-03):
- HelmRelease (chart 8.x / Velero 1.15) + velero-plugin-for-aws init container +
  node-agent kopia File System Backup (deployNodeAgent, uploaderType=kopia,
  snapshotsEnabled=false). BSL default -> bucket velero, s3ForcePathStyle.
- Two daily Schedules: `daily` backs up the whole cluster's resources+PVC data
  (defaultVolumesToFsBackup opt-out) EXCEPT monitoring+unifi; `daily-resources-only`
  captures monitoring+unifi RESOURCES only (Prometheus TSDB, Loki 72h logs, and the
  ~7.4GiB UniFi VM disk are reconstructable — excluded from volume backup).
- velero-cloud-credentials SOPS Secret (cloud INI key, ns velero).
- Own non-gating Flux tier clusters/base/backup.yaml (dependsOn platform-config,
  wait:false) + clusters/dev SOPS decryption patch — mirrors headlamp.

CNPG native Barman PITR (R-IDP-05 / R-STATE-03):
- keycloak-db, nextcloud-db, semaphore-db gain spec.backup.barmanObjectStore
  (s3://cnpg/<cluster>, endpointURL MinIO, gzip WAL+data, 30d retention) + a daily
  ScheduledBackup (6-field cron, immediate) each.
- Per-namespace scoped-key SOPS Secrets (<db>-backup-credentials), same-ns only.
- render.py: emit-waves now recognizes <svc>-db-backup-credentials.sops.yaml as a
  DB-wave sidecar (+ self-test); wave-0 kustomization regenerated.

Creds mirror platform/backup/minio-credentials.sops.yaml (scoped key k3s-backup),
SOPS round-trip verified with the cluster age key. gate: green.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

clusters/base/kustomization.yaml

@@ -34,4 +34,8 @@ resources:
   # reverse-proxy sidecar (Option B). POST-platform tier (dependsOn platform-config, wait:false); same
   # non-gating leaf-UI rationale as headlamp/homepage/oauth2-proxy (a UI must never gate the platform).
   - crafty.yaml          # Flux Kustomization -> ./platform/crafty (dependsOn platform-config)
+  # Velero backup controller — POST-platform tier (dependsOn platform-config, wait:false). A backup
+  # controller must NEVER gate the platform chain; same non-gating rationale as headlamp/oauth2-proxy.
+  # Carries velero-cloud.sops.yaml, so clusters/dev adds a SOPS decryption patch for it (by name).
+  - backup.yaml          # Flux Kustomization -> ./platform/velero (dependsOn platform-config)
   - cluster-vars.yaml    # overlay knob surface (base defaults)