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_EN.md

@@ -15,6 +15,7 @@
 
 ### Bug Fixes
 
+- fix: |Telegram| Fix admin users unable to view emails via Telegram MiniApp due to `Auth date expired` error, support admin password auth for viewing emails
 - fix: |Admin API| Fix `/admin/account_settings` throwing `Cannot read properties of undefined (reading 'put')` when KV is not configured and `fromBlockList` is empty
 - fix: |Database| Fix missing `idx_raw_mails_message_id` index in `DB_INIT_QUERIES` causing full table scan on `UPDATE raw_mails ... WHERE message_id = ?`, sync `schema.sql` with init code, add v0.0.6 migration
 - fix: |Docs| Fix User Mail API documentation incorrectly using `x-admin-auth`, changed to correct `x-user-token`