[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/headscale/main.tf

@@ -92,6 +92,27 @@ resource "kubernetes_deployment" "headscale" {
             container_port = 41641
           }
 
+          liveness_probe {
+            http_get {
+              path = "/health"
+              port = 8080
+            }
+            initial_delay_seconds = 15
+            period_seconds        = 30
+            timeout_seconds       = 5
+            failure_threshold     = 5
+          }
+          readiness_probe {
+            http_get {
+              path = "/health"
+              port = 8080
+            }
+            initial_delay_seconds = 5
+            period_seconds        = 30
+            timeout_seconds       = 5
+            failure_threshold     = 3
+          }
+
           volume_mount {
             name       = "config-volume"
             mount_path = "/etc/headscale"