fix: support admin auth for Telegram MiniApp mail viewing (#875)

* fix: support admin auth for Telegram MiniApp mail viewing (#852)

Admin users configured in miniAppUrl can now view emails via the
MiniApp without needing Telegram initData auth. The getMail endpoint
reads the x-admin-auth header (already sent by the frontend) to
bypass Telegram auth and address permission checks for admin users.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: extract isAdmin() as shared utility function

Reuse the x-admin-auth header check logic across worker.ts and
miniapp.ts via a common isAdmin() helper in utils.ts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: rename isAdmin to checkIsAdmin for consistency

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address PR review comments

- Remove unused getAdminPasswords import from worker.ts
- Return 404 when admin queries a non-existent mail

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

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

CHANGELOG.md

@@ -15,6 +15,7 @@
 
 ### Bug Fixes
 
+- fix: |Telegram| 修复 admin 用户通过 Telegram MiniApp 查看邮件时报 `Auth date expired` 的问题，支持 admin 密码认证查看邮件
 - fix: |Admin API| 修复 `/admin/account_settings` 在未配置 KV 且 `fromBlockList` 为空时触发 `Cannot read properties of undefined (reading 'put')` 的问题
 - fix: |数据库| 修复 `DB_INIT_QUERIES` 缺少 `idx_raw_mails_message_id` 索引导致 `UPDATE raw_mails ... WHERE message_id = ?` 全表扫描的问题，同步 `schema.sql` 与初始化代码，新增 v0.0.6 迁移逻辑
 - fix: |文档| 修复 User Mail API 文档中错误使用 `x-admin-auth` 的问题，改为正确的 `x-user-token`