fix(postfix): use hash: transport_maps with postmap init container

- Splits hash-type map files into a separate postfix-maps ConfigMap
- Adds postmap init container that builds .db files from all maps into
  a postfix-db emptyDir, which is then subPath-mounted per map in the
  main container
- Updates transport_maps in main.cf to hash:/etc/postfix/transport

apps/base/mailgateway/resources/postfix/main.cf

@@ -13,9 +13,8 @@ alias_maps =
 alias_database =
 
 # Relay inbound mail for these domains to Stalwart
-# texthash: reads plain text without requiring postmap (Alpine has no hash/btree)
 relay_domains = main.unkin.net unkin.net
-transport_maps = texthash:/etc/postfix/transport
+transport_maps = hash:/etc/postfix/transport
 
 # rspamd milter (same namespace — short DNS name resolves)
 smtpd_milters = inet:rspamd:11332