feat(mail-admin): read-only account view for mail.produktor.io (#74)

- admin/: Go stdlib-only HTTP viewer, lists accounts from
  config/postfix-accounts.cf with per-mailbox message counts (INBOX and
  total, same numbers doveadm reports) and storage; quota from
  dovecot-quotas.cf; Basic Auth from .env; offline tests vs fixtures
  (go test -race ./...)
- compose: mail-admin service, build admin/Dockerfile, publishes
  127.0.0.1:19945 / 172.17.0.1:19945; config and mail-data mounted :ro,
  no docker socket
- NPM proxy host 66: location /admin/ -> 172.17.0.1:19945
- README: account admin section

compose.yaml

@@ -59,3 +59,32 @@ services:
       - ROUNDCUBEMAIL_USERNAME_DOMAIN=produktor.io
       - ROUNDCUBEMAIL_SKIN=elastic
       - ROUNDCUBEMAIL_DES_KEY=${ROUNDCUBEMAIL_DES_KEY:?set ROUNDCUBEMAIL_DES_KEY in .env}
+
+  # Account admin (read-only view) — https://mail.produktor.io/admin/ (NPM proxy
+  # host 66, location /admin/ -> 172.17.0.1:19945).
+  # Lists the accounts from config/postfix-accounts.cf with per-mailbox message
+  # counts (INBOX / total) and storage, computed the same way doveadm reports
+  # them: every file in a mailbox's cur/ or new/ directory is one message.
+  # Read-only: config and mail data are mounted with :ro, no docker socket.
+  mail-admin:
+    build:
+      context: ./admin
+    image: mail-admin:local
+    container_name: mail-admin
+    restart: unless-stopped
+    depends_on:
+      - mailserver
+    ports:
+      - "127.0.0.1:19945:8080"
+      - "172.17.0.1:19945:8080"
+    volumes:
+      - ./config/:/config/:ro
+      - ./data/mail-data/:/var/mail/:ro
+    environment:
+      - MAIL_ADMIN_LISTEN=:8080
+      - MAIL_ADMIN_BASE_PATH=/admin
+      - MAIL_ADMIN_ACCOUNTS=/config/postfix-accounts.cf
+      - MAIL_ADMIN_QUOTAS=/config/dovecot-quotas.cf
+      - MAIL_ADMIN_MAILDIR=/var/mail
+      - MAIL_ADMIN_USER=${MAIL_ADMIN_USER:?set MAIL_ADMIN_USER in .env}
+      - MAIL_ADMIN_PASSWORD=${MAIL_ADMIN_PASSWORD:?set MAIL_ADMIN_PASSWORD in .env}