feat(postfix): deploy postfix MTA and rspamd spam filter

- mailgateway namespace with Deployment + HPA (2-6 replicas)
- rspamd Deployment + HPA (2-6 replicas) with milter interface
- postfix configured to relay inbound mail to stalwart via transport maps
- rspamd milter on port 11332 for spam scanning and DKIM signing
- DKIM keys stored in Vault at kubernetes/namespace/mailgateway/default/dkim-keys
- TLS cert via cert-manager (vault-issuer) for mail.main.unkin.net
- rspamd web UI exposed via Traefik Gateway at rspamd.k8s.syd1.au.unkin.net
- postfix external LoadBalancer service for inbound MX on port 25
- Add full main.cf and master.cf as ConfigMap resources mounted via subPath
  - main.cf: relay-only gateway config, texthash: transport maps, rspamd milter
  - master.cf: standard smtp + submission (587, TLS required) + internal processes
- MAILNAME/MY_NETWORKS/MY_DESTINATION env vars kept in sync with main.cf
- LOG_TO_STDOUT=1 for k8s log collection

apps/base/mailgateway/gateway.yaml

@@ -0,0 +1,37 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  labels:
+    traefik.io/instance: internal
+  annotations:
+    cert-manager.io/cluster-issuer: vault-issuer
+    cert-manager.io/common-name: rspamd.k8s.syd1.au.unkin.net
+    cert-manager.io/private-key-size: "4096"
+    external-dns.alpha.kubernetes.io/hostname: rspamd.k8s.syd1.au.unkin.net
+    external-dns.alpha.kubernetes.io/target: 198.18.200.4
+  name: rspamd
+  namespace: mailgateway
+spec:
+  gatewayClassName: traefik-internal
+  listeners:
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: rspamd.k8s.syd1.au.unkin.net
+      name: http
+      port: 80
+      protocol: HTTP
+    - allowedRoutes:
+        namespaces:
+          from: Same
+      hostname: rspamd.k8s.syd1.au.unkin.net
+      name: https
+      port: 443
+      protocol: HTTPS
+      tls:
+        certificateRefs:
+          - group: ""
+            kind: Secret
+            name: rspamd-tls
+        mode: Terminate