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.md

@@ -19,6 +19,8 @@
 - feat: |邮件过滤| 移除后端 keyword 参数，改为前端过滤当前页邮件，优化查询性能
 - feat: |数据库| 为 `message_id` 字段添加索引，优化邮件更新操作性能，需执行 `db/2025-12-15-message-id-index.sql` 更新数据库
 - feat: |Admin| 维护页面增加自定义 SQL 清理功能，支持定时任务执行自定义清理语句
+- feat: |国际化| 后端 API 错误消息全面支持中英文国际化
+- feat: |Telegram| 机器人支持中英文切换，新增 `/lang` 命令设置语言偏好
 
 ## v1.1.0