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/k8s/listmonk/pvc.yaml

@@ -0,0 +1,12 @@
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: listmonk-uploads
+  namespace: listmonk
+spec:
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 5Gi