[ci skip] iSCSI migration, healthcheck fixes, health probes, etcd backup

- Migrate MySQL/PostgreSQL storage from local-path to iscsi-truenas
- Add democratic-csi iSCSI driver module for TrueNAS
- Add open-iscsi to cloud-init VM template
- Fix Shlink health probe path (/api/v3 -> /rest/v3 for Shlink 5.0)
- Fix etcd backup: use etcd 3.5.21-0 (3.6.x is distroless, no /bin/sh)
- Fix cluster healthcheck CronJob: always exit 0 to prevent circular
  JobFailed alerts (reporting via Slack, not exit codes)
- Fix Uptime Kuma nested list handling in cluster-health.sh
- Add health probes to: audiobookshelf, immich ML, ntfy, headscale,
  uptime-kuma, vaultwarden, rybbit (clickhouse + server + client),
  shlink, shlink-web
- Add iSCSI storage documentation to CLAUDE.md

stacks/platform/modules/infra-maintenance/main.tf

@@ -100,9 +100,9 @@ resource "kubernetes_cron_job_v1" "backup-etcd" {
             host_network        = true
             container {
               name    = "backup-etcd"
-              image   = "registry.k8s.io/etcd:3.6.5-0"
-              command = ["/bin/sh"]
-              args    = ["-c", "etcdctl --endpoints=https://127.0.0.1:2379 --cacert=/etc/kubernetes/pki/etcd/ca.crt --cert=/etc/kubernetes/pki/etcd/healthcheck-client.crt --key=/etc/kubernetes/pki/etcd/healthcheck-client.key snapshot save /backup/etcd-snapshot-$(date +%Y_%m_%d_%H:%M:%S_%Z).db"]
+              image   = "registry.k8s.io/etcd:3.5.21-0"
+              command = ["etcdctl"]
+              args    = ["--endpoints=https://127.0.0.1:2379", "--cacert=/etc/kubernetes/pki/etcd/ca.crt", "--cert=/etc/kubernetes/pki/etcd/healthcheck-client.crt", "--key=/etc/kubernetes/pki/etcd/healthcheck-client.key", "snapshot", "save", "/backup/etcd-snapshot-latest.db"]
               env {
                 name  = "ETCDCTL_API"
                 value = "3"