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/apps/listmonk.yaml

@@ -0,0 +1,20 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: listmonk
+  namespace: argocd
+spec:
+  project: default
+  source:
+    repoURL: https://git.neuralplatform.ai/will/infrastructure.git
+    targetRevision: main
+    path: servers/legion/k8s/listmonk
+  destination:
+    server: https://kubernetes.default.svc
+    namespace: listmonk
+  syncPolicy:
+    automated:
+      prune: true
+      selfHeal: true
+    syncOptions:
+      - CreateNamespace=true