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- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap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/certificate.yaml

@@ -0,0 +1,18 @@
+---
+apiVersion: cert-manager.io/v1
+kind: Certificate
+metadata:
+  name: postfix-smtp-tls
+  namespace: mailgateway
+spec:
+  secretName: postfix-smtp-tls
+  issuerRef:
+    name: vault-issuer
+    kind: ClusterIssuer
+  commonName: mail.main.unkin.net
+  dnsNames:
+    - mail.main.unkin.net
+    - smtp-in.main.unkin.net
+  privateKey:
+    size: 4096
+    algorithm: RSA