feat: add Telegram proxy support and flexible allow_from matching

- Add proxy config field for Telegram channel to support HTTP/SOCKS proxies
- Use telego.WithHTTPClient to route all Telegram API requests through proxy
- Add FlexibleStringSlice type so allow_from accepts both strings and numbers
- Improve IsAllowed to match numeric ID, username, and @username formats
- Update config.example.json with proxy field

config.example.json

@@ -13,6 +13,7 @@
     "telegram": {
       "enabled": false,
       "token": "YOUR_TELEGRAM_BOT_TOKEN",
+      "proxy": "",
       "allow_from": ["YOUR_USER_ID"]
     },
     "discord": {