feat(mail): 增强邮件限流与配额配置

- 新增 IMAP/POP3 事件表与清理任务，并接入 Dovecot auth policy。
- 暴露默认权限限额接口，前端权限组页面改为动态读取。
- 为 Dovecot 配置用户配额、连接数限制和 auth policy。

deploy/dovecot/dovecot.conf

@@ -10,6 +10,20 @@ 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
+mail_max_userip_connections = 10
+
+plugin {
+  quota = maildir:User quota
+  quota_rule = *:storage=1G
+  # auth_policy_server is configured via dovecot-sql.conf.ext user_query
+}
+
+auth_policy_server_url = http://127.0.0.1:8080/auth-policy
+auth_policy_server_api_header = Content-Type: application/json
+auth_policy_hash_mech = sha256
+auth_policy_hash_truncate = 12
+auth_policy_request_fields = protocol=imap username=user
+
 namespace inbox {
   inbox = yes
   mailbox Drafts {
@@ -38,6 +52,14 @@ userdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
 }
 
+protocol imap {
+  mail_plugins = quota imap_quota
+}
+
+protocol pop3 {
+  mail_plugins = quota
+}
+
 service imap-login {
   inet_listener imaps {
     port = 993