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- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap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

argocd/applicationsets/platform.yaml

@@ -21,7 +21,7 @@ spec:
       - path: apps/overlays/*/inteldeviceplugins-system
       - path: apps/overlays/*/jfrog
       - path: apps/overlays/*/node-feature-discovery
-      - path: apps/overlays/*/postfix
+      - path: apps/overlays/*/mailgateway
       - path: apps/overlays/*/puppet
       - path: apps/overlays/*/purelb
       - path: apps/overlays/*/reflector-system