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- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap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/artifactapi/resources/conf.d/remote-docker.yaml

@@ -20,7 +20,6 @@ remotes:
     package: "docker"
     description: "Docker Hub registry"
     immutable_patterns:
-      - "^boky/postfix"
       - "^library/almalinux"
       - "^library/busybox"
       - "^library/debian"
@@ -37,6 +36,7 @@ remotes:
       - "^jfrog/"
       - "^rancher/"
       - "^rspamd/rspamd"
+      - "^tozd/postfix"
       - "^traefik/"
       - "^ubi9/ubi-minimal"
       - "^victoriametrics/"