feat(deploy): 支持配置邮件客户端 TLS 证书

- 新增 `LANQIN_TLS_CERT_FILE` 和 `LANQIN_TLS_KEY_FILE`，让 Postfix 与 Dovecot 使用外部证书。
- 在单容器与分服务启动脚本中统一回退到 `localhost` 自签证书，并在证书不可读时给出告警。
- 补充部署文档与示例配置，说明第三方客户端校验证书的挂载方式。

deploy/dovecot/entrypoint.sh

@@ -1,7 +1,21 @@
 #!/bin/sh
 set -eu
+: "${LANQIN_TLS_CERT_FILE:=}"
+: "${LANQIN_TLS_KEY_FILE:=}"
 addgroup --system --gid 5000 vmail 2>/dev/null || true
 adduser --system --uid 5000 --gid 5000 --home /var/mail/vhosts --no-create-home vmail 2>/dev/null || true
 mkdir -p /var/mail/vhosts
 chown -R 5000:5000 /var/mail/vhosts
+TLS_CERT=/etc/ssl/certs/ssl-cert-snakeoil.pem
+TLS_KEY=/etc/ssl/private/ssl-cert-snakeoil.key
+if [ -n "$LANQIN_TLS_CERT_FILE" ] || [ -n "$LANQIN_TLS_KEY_FILE" ]; then
+  if [ -f "$LANQIN_TLS_CERT_FILE" ] && [ -f "$LANQIN_TLS_KEY_FILE" ]; then
+    TLS_CERT="$LANQIN_TLS_CERT_FILE"
+    TLS_KEY="$LANQIN_TLS_KEY_FILE"
+  else
+    echo "warning: LANQIN_TLS_CERT_FILE/LANQIN_TLS_KEY_FILE not readable; using snakeoil localhost certificate" >&2
+  fi
+fi
+sed -i "s#^ssl_cert = <.*#ssl_cert = <${TLS_CERT}#" /etc/dovecot/dovecot.conf
+sed -i "s#^ssl_key = <.*#ssl_key = <${TLS_KEY}#" /etc/dovecot/dovecot.conf
 exec dovecot -F