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- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap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

@@ -0,0 +1,32 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - certificate.yaml
+  - gateway.yaml
+  - httproute.yaml
+  - namespace.yaml
+  - postfix-deployment.yaml
+  - postfix-hpa.yaml
+  - rspamd-deployment.yaml
+  - rspamd-hpa.yaml
+  - services.yaml
+  - vaultauth.yaml
+  - vaultstaticsecret.yaml
+
+configMapGenerator:
+  - name: postfix-config
+    files:
+      - main.cf=resources/postfix/main.cf
+      - master.cf=resources/postfix/master.cf
+      - transport=resources/postfix/transport
+    options:
+      disableNameSuffixHash: true
+  - name: rspamd-config
+    files:
+      - worker-proxy.inc=resources/rspamd/local.d/worker-proxy.inc
+      - dkim_signing.conf=resources/rspamd/local.d/dkim_signing.conf
+      - milter_headers.conf=resources/rspamd/local.d/milter_headers.conf
+    options:
+      disableNameSuffixHash: true