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/ingress.yaml

@@ -0,0 +1,26 @@
+---
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: listmonk
+  namespace: listmonk
+  annotations:
+    traefik.ingress.kubernetes.io/router.entrypoints: websecure
+    cert-manager.io/cluster-issuer: letsencrypt-prod
+spec:
+  ingressClassName: traefik
+  tls:
+    - hosts:
+        - newsletter.harmonic-framework.com
+      secretName: listmonk-tls
+  rules:
+    - host: newsletter.harmonic-framework.com
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: listmonk
+                port:
+                  number: 9000