refactor(mailgateway): rename namespace postfix→mailgateway, switch to tozd/postfix

- Rename apps/base/postfix → apps/base/mailgateway
- Rename apps/overlays/au-syd1/postfix → apps/overlays/au-syd1/mailgateway
- Namespace postfix → mailgateway throughout all resources
- Replace boky/postfix with tozd/postfix:alpine-322 (Postfix 3.10.2 on Alpine 3.22)
- 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
- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap
- MAILNAME/MY_NETWORKS/MY_DESTINATION env vars kept in sync with main.cf
- LOG_TO_STDOUT=1 for k8s log collection
- Vault DKIM keys now at kubernetes/namespace/mailgateway/default/dkim-keys
- artifactapi: replace boky/postfix with tozd/postfix in dockerhub cache

apps/base/mailgateway/services.yaml

@@ -0,0 +1,61 @@
+---
+# Internal service for rspamd - used by postfix pods in the same namespace
+apiVersion: v1
+kind: Service
+metadata:
+  name: rspamd
+  namespace: mailgateway
+spec:
+  selector:
+    app: rspamd
+  ports:
+    - name: milter
+      port: 11332
+      targetPort: 11332
+      protocol: TCP
+    - name: worker
+      port: 11333
+      targetPort: 11333
+      protocol: TCP
+    - name: controller
+      port: 11334
+      targetPort: 11334
+      protocol: TCP
+---
+# Internal ClusterIP for postfix - used by stalwart for outbound relay
+apiVersion: v1
+kind: Service
+metadata:
+  name: postfix
+  namespace: mailgateway
+spec:
+  selector:
+    app: postfix
+  ports:
+    - name: smtp
+      port: 25
+      targetPort: 25
+      protocol: TCP
+    - name: submission
+      port: 587
+      targetPort: 587
+      protocol: TCP
+---
+# External LoadBalancer for inbound MX (internet → postfix)
+apiVersion: v1
+kind: Service
+metadata:
+  name: postfix-external
+  namespace: mailgateway
+  annotations:
+    external-dns.alpha.kubernetes.io/hostname: smtp-in.main.unkin.net
+spec:
+  type: LoadBalancer
+  externalTrafficPolicy: Local
+  selector:
+    app: postfix
+  ports:
+    - name: smtp
+      port: 25
+      targetPort: 25
+      protocol: TCP