[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

modules/create-template-vm/cloud_init.yaml

@@ -38,6 +38,8 @@ packages:
   # kubernetes
   - kubeadm
   - kubelet
+  # iSCSI client for CSI-backed database storage
+  - open-iscsi
   %{endif}
 
 apt:
@@ -60,6 +62,7 @@ runcmd:
   - containerd config default | sudo tee /etc/containerd/config.toml
   - ${containerd_config_update_command}
   - systemctl restart containerd
+  - systemctl enable --now iscsid
   - ${k8s_join_command}
   - systemctl enable kubelet
   - systemctl start kubelet