refactor: modularize IMAP server with dual login, STARTTLS, and test suite (#859)

refactor: modularize IMAP server with fixes and E2E tests

- Modularize IMAP server into imap_server, imap_mailbox, imap_message,
  imap_http_client, parse_email, config, models
- Support dual login: JWT token and address+password via backend
- Add STARTTLS support with configurable TLS cert/key
- Fix FETCH/STORE returning UID instead of sequence number (RFC 3501)
- Implement IMessageFile.open() for correct BODY[] raw MIME delivery
- Add UIDNEXT to SELECT response via _cbSelectWork override
- Use per-restart UIDVALIDITY to force client resync
- Pass raw MIME to SimpleMessage for accurate RFC822.SIZE
- Fix SENT mailbox returning empty source
- Handle CREATE command gracefully for Thunderbird compatibility
- Add IMAP E2E tests: auth, LIST, SELECT, STATUS, FETCH, SEARCH,
  STORE, UID FETCH, BODY[] integrity, size, seq numbers, SENT mailbox
- Add SMTP E2E tests using nodemailer: send plain/HTML, auth failure,
  sendbox verification
- Add sendTestMail helper using admin/send_mail

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -35,6 +35,9 @@
 - style: |邮件列表| 优化收件箱和发件箱空状态显示，根据邮件数量显示不同提示信息，添加语义化图标
 - feat: |后台管理| 邮箱地址列表来源IP添加 ip.im 查询链接，点击可快速查看IP信息
 - docs: |文档| 修复 VitePress 中英文切换路径错误，改用双前缀 locale 配置
+- feat: |IMAP 代理| 重构 IMAP 服务端，拆分为独立模块（HTTP 客户端、邮箱、消息），使用 `deferToThread` 异步 HTTP 避免阻塞 Twisted reactor，使用后端 `id` 作为稳定 UID，新增 STARTTLS 支持、LRU 消息缓存、session 级 flags 管理、SEARCH 命令支持、JWT 凭证和地址+密码双登录方式，新增完整测试套件
+- fix: |IMAP 代理| 修复 `getHeaders()` 过滤逻辑、`store()` 崩溃问题
+- fix: |邮件解析| 修复 `parse_email.py` 中使用私有属性 `_payload` 导致编码错误的问题，改用 `get_payload(decode=True)` 正确解码邮件体
 
 ## v1.3.0