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.conf

@@ -0,0 +1,48 @@
+protocols = imap lmtp
+listen = *
+base_dir = /var/run/dovecot/
+
+log_path = /dev/stderr
+info_log_path = /dev/stdout
+
+ssl = yes
+ssl_cert = </etc/ssl/certs/ssl-cert-snakeoil.pem
+ssl_key = </etc/ssl/private/ssl-cert-snakeoil.key
+
+mail_home = /var/mail/vhosts/%d/%n
+namespace inbox {
+  inbox = yes
+  mailbox Drafts { special_use = \Drafts }
+  mailbox Sent { special_use = \Sent }
+  mailbox Trash { special_use = \Trash }
+  mailbox Archive { special_use = \Archive }
+  mailbox Spam { special_use = \Junk }
+}
+
+passdb {
+  driver = sql
+  args = /etc/dovecot/dovecot-sql.conf.ext
+}
+userdb {
+  driver = sql
+  args = /etc/dovecot/dovecot-sql.conf.ext
+}
+
+service imap-login {
+  inet_listener imaps { port = 993 ssl = yes }
+}
+
+service lmtp {
+  inet_listener lmtp { address = 0.0.0.0 port = 24 }
+}
+
+service auth {
+  inet_listener postfix-auth {
+    address = 0.0.0.0
+    port = 12345
+  }
+}
+
+protocol lmtp {
+  postmaster_address = postmaster@localhost
+}