feat: add Listmonk newsletter platform on Legion k3s

- Namespace, ExternalSecrets (Vault), DB init job, deployment, service, PVC, ingress
- Routes newsletter.harmonic-framework.com via existing Cloudflare tunnel
- Uses shared platform postgres with dedicated listmonk DB/user
- Credentials stored at secret/data/listmonk in Vault

servers/legion/namespaces.tf

@@ -47,3 +47,13 @@ resource "kubernetes_namespace" "harmonic_wordpress" {
     }
   }
 }
+
+resource "kubernetes_namespace" "listmonk" {
+  metadata {
+    name = "listmonk"
+    labels = {
+      managed-by = "terraform"
+      tier       = "apps"
+    }
+  }
+}