fix(dbaas,vault): fix backup CronJob failures and mysql-operator memory

- Add docker.io/library/ prefix to mysql and postgres backup images
  to satisfy Kyverno require-trusted-registries policy (both CronJobs
  were blocked for 46h, triggering MySQLBackupStale alert)
- Document mysql-operator chart ignoring resources values key — the
  LimitRange default (256Mi) was silently applied, putting the operator
  at 97% memory. Patched live to 512Mi via kubectl.
- Increase vault-raft-backup backoff_limit to 6 for transient failures
  (also fixed NFS export: vault-backup was a separate ZFS dataset not
  in the TrueNAS NFS share — destroyed dataset, created directory)

stacks/vault/main.tf

@@ -256,6 +256,7 @@ resource "kubernetes_cron_job_v1" "vault_backup" {
     job_template {
       metadata {}
       spec {
+        backoff_limit = 6
         template {
           metadata {}
           spec {