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/opendkim/Dockerfile

@@ -0,0 +1,9 @@
+FROM debian:bookworm-slim
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && apt-get install -y --no-install-recommends opendkim opendkim-tools sqlite3 ca-certificates && rm -rf /var/lib/apt/lists/*
+COPY opendkim.conf /etc/opendkim.conf
+COPY TrustedHosts /etc/opendkim/TrustedHosts
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+EXPOSE 8891
+CMD ["/entrypoint.sh"]

deploy/opendkim/TrustedHosts

@@ -0,0 +1,5 @@
+127.0.0.1
+localhost
+10.0.0.0/8
+172.16.0.0/12
+192.168.0.0/16

deploy/opendkim/entrypoint.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+set -eu
+mkdir -p /etc/opendkim/keys
+: > /etc/opendkim/KeyTable
+: > /etc/opendkim/SigningTable
+if [ -f /data/lanqin.db ]; then
+  sqlite3 -separator '|' /data/lanqin.db "SELECT name, dkim_selector, dkim_private_key FROM domains WHERE status='active';" | while IFS='|' read -r domain selector private_key; do
+    [ -n "$domain" ] || continue
+    dir="/etc/opendkim/keys/$domain"
+    mkdir -p "$dir"
+    keyfile="$dir/$selector.private"
+    printf '%s' "$private_key" | base64 -d > "$keyfile"
+    chmod 600 "$keyfile"
+    echo "$selector._domainkey.$domain $domain:$selector:$keyfile" >> /etc/opendkim/KeyTable
+    echo "*@${domain} ${selector}._domainkey.${domain}" >> /etc/opendkim/SigningTable
+  done
+fi
+chown -R opendkim:opendkim /etc/opendkim
+exec opendkim -f -x /etc/opendkim.conf

deploy/opendkim/opendkim.conf

@@ -0,0 +1,12 @@
+Syslog                  yes
+UMask                   002
+Mode                    sv
+Canonicalization        relaxed/simple
+SubDomains              no
+OversignHeaders         From
+Socket                  inet:8891@0.0.0.0
+UserID                  opendkim:opendkim
+KeyTable                /etc/opendkim/KeyTable
+SigningTable            refile:/etc/opendkim/SigningTable
+ExternalIgnoreList      /etc/opendkim/TrustedHosts
+InternalHosts           /etc/opendkim/TrustedHosts