feat(email): 初始化自建邮箱 MVP

- 新增 Go + SQLite 后端，支持登录、域名、邮箱、别名、联系人、规则、统计与邮件收发。
- 新增 Webmail 前端，支持多邮箱切换、邮件列表/阅读/写信、附件、搜索、主题切换与个人中心。
- 新增 Docker Compose 部署骨架，补充 Postfix、Dovecot、OpenDKIM、Nginx 配置与部署说明。

deploy/dovecot/dovecot-sql.conf.ext

@@ -0,0 +1,5 @@
+driver = sqlite
+connect = /data/lanqin.db
+default_pass_scheme = BLF-CRYPT
+password_query = SELECT address AS user, password_hash AS password FROM mailboxes WHERE address = '%u' AND status = 'active'
+user_query = SELECT '/var/mail/vhosts/' || substr(address, instr(address, '@') + 1) || '/' || local_part AS home, 'maildir:/var/mail/vhosts/' || substr(address, instr(address, '@') + 1) || '/' || local_part || '/Maildir' AS mail, 5000 AS uid, 5000 AS gid FROM mailboxes WHERE address = '%u' AND status = 'active'