chore(deploy): 配置已发送邮件自动归档

- 为 `submission` 和 `smtps` 启用 `sender_bcc_maps`，将已认证发件人自动抄送到 `发件人+Sent@域名`。
- 补充 Postfix 与 Dovecot 的 `recipient_delimiter` 和 `lmtp_save_to_detail_mailbox` 配置，确保 `Sent` 文件夹可落盘保存。
- 更新部署文档，说明第三方客户端发信后的“已发送”同步流程。

deploy/postfix/main.cf

@@ -4,6 +4,7 @@ myorigin = $myhostname
 mydestination = localhost
 inet_interfaces = all
 inet_protocols = all
+recipient_delimiter = +
 
 # SQLite maps read the same LanQin DB created by the Go API.
 virtual_mailbox_domains = sqlite:/etc/postfix/sqlite-domains.cf

deploy/postfix/master.cf

@@ -4,11 +4,13 @@ submission inet n       -       n       -       -       smtpd
   -o smtpd_tls_security_level=may
   -o smtpd_sasl_auth_enable=yes
   -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
+  -o sender_bcc_maps=sqlite:/etc/postfix/sqlite-sender-bcc.cf
 smtps     inet  n       -       n       -       -       smtpd
   -o syslog_name=postfix/smtps
   -o smtpd_tls_wrappermode=yes
   -o smtpd_sasl_auth_enable=yes
   -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
+  -o sender_bcc_maps=sqlite:/etc/postfix/sqlite-sender-bcc.cf
 pickup    unix  n       -       n       60      1       pickup
 cleanup   unix  n       -       n       -       0       cleanup
 qmgr      unix  n       -       n       300     1       qmgr

deploy/postfix/sqlite-sender-bcc.cf

@@ -0,0 +1,2 @@
+dbpath = /data/lanqin.db
+query = SELECT local_part || '+Sent@' || substr(address, instr(address, '@') + 1) FROM mailboxes WHERE address='%s' AND status='active'