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- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap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/resources/rspamd/local.d/dkim_signing.conf

@@ -0,0 +1,13 @@
+enabled = true;
+selector = "mail";
+
+domain {
+    main.unkin.net {
+        privkey = "/etc/rspamd/dkim/private_key";
+        selector = "mail";
+    }
+    unkin.net {
+        privkey = "/etc/rspamd/dkim/private_key";
+        selector = "mail";
+    }
+}

apps/base/mailgateway/resources/rspamd/local.d/milter_headers.conf

@@ -0,0 +1,2 @@
+extended_spam_headers = true;
+use = ["x-spam-status", "x-spam-score", "authentication-results"];

apps/base/mailgateway/resources/rspamd/local.d/worker-proxy.inc

@@ -0,0 +1,7 @@
+milter = yes;
+bind_socket = "*:11332";
+
+upstream "local" {
+    default = yes;
+    self_scan = yes;
+}