feat: add i18n support for backend API and Telegram bot (#797)

* feat: add i18n support for backend API and Telegram bot

- Add comprehensive i18n support for all backend API error messages (zh/en)
- Add /lang command for Telegram bot to set language preference
- Add bilingual command descriptions for Telegram bot
- Support per-user language preference stored in KV
- Global push uses DEFAULT_LANG, user push uses saved preference

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* fix: improve Telegram bot language preference feature

- Add internationalized message for disabled language feature
- Fix hardcoded English message in /lang command
- Optimize getTgMessages calls (reduce from 3 to 1 call)
- Remove verbose comments for better code clarity
- Add TgLangFeatureDisabledMsg to i18n (zh/en)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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

CHANGELOG_EN.md

@@ -19,6 +19,8 @@
 - feat: |Email Filtering| Remove backend keyword parameter, switch to frontend filtering of current page emails, optimize query performance
 - feat: |Database| Add index for `message_id` field to optimize email update operations, need to execute `db/2025-12-15-message-id-index.sql` to update database
 - feat: |Admin| Add custom SQL cleanup feature to maintenance page, support scheduled task execution of custom cleanup statements
+- feat: |i18n| Backend API error messages now fully support Chinese and English internationalization
+- feat: |Telegram| Bot supports Chinese/English switching, add `/lang` command to set language preference
 
 ## v1.1.0