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- Remove custom.cf (superseded by main.cf); merge transport into postfix-config ConfigMap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

@@ -0,0 +1,42 @@
+# ==========================================================================
+# service type  private unpriv  chroot  wakeup  maxproc command + args
+#               (yes)   (yes)   (yes)   (never) (100)
+# ==========================================================================
+
+# SMTP inbound (port 25) — runs rspamd milter, relays to Stalwart via transport_maps
+smtp      inet  n       -       n       -       -       smtpd
+
+# Submission (port 587) — TLS required, relay from trusted mynetworks only
+submission inet n       -       n       -       -       smtpd
+  -o syslog_name=postfix/submission
+  -o smtpd_tls_security_level=encrypt
+  -o smtpd_sasl_auth_enable=no
+  -o smtpd_reject_unlisted_recipient=no
+  -o smtpd_relay_restrictions=permit_mynetworks,reject
+  -o milter_macro_daemon_name=ORIGINATING
+
+# Internal postfix processes
+pickup    unix  n       -       n       60      1       pickup
+cleanup   unix  n       -       n       -       0       cleanup
+qmgr      unix  n       -       n       300     1       qmgr
+tlsmgr    unix  -       -       n       1000?   1       tlsmgr
+rewrite   unix  -       -       n       -       -       trivial-rewrite
+bounce    unix  -       -       n       -       0       bounce
+defer     unix  -       -       n       -       0       bounce
+trace     unix  -       -       n       -       0       bounce
+verify    unix  -       -       n       -       1       verify
+flush     unix  n       -       n       1000?   0       flush
+proxymap  unix  -       -       n       -       -       proxymap
+proxywrite unix -       -       n       -       1       proxymap
+smtp      unix  -       -       n       -       -       smtp
+relay     unix  -       -       n       -       -       smtp
+showq     unix  n       -       n       -       -       showq
+error     unix  -       -       n       -       -       error
+retry     unix  -       -       n       -       -       error
+discard   unix  -       -       n       -       -       discard
+local     unix  -       n       n       -       -       local
+virtual   unix  -       n       n       -       -       virtual
+lmtp      unix  -       -       n       -       -       lmtp
+anvil     unix  -       -       n       -       1       anvil
+scache    unix  -       -       n       -       1       scache
+postlog   unix-dgram n  -       n       -       1       postlogd