fix(bot): register webhook with Telegram on startup
Previously, webhook mode only logged the URL but never actually called
setWebhook. This meant the secret_token was never configured, causing
Telegram to send updates without the X-Telegram-Bot-Api-Secret-Token
header, which the app then rejected with 401 Unauthorized.
Now registerWebhook() is called on startup in webhook mode, ensuring
the webhook is properly configured with the secret token.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>