Use Telegram's sendVoice API for audio files by default, displaying them
as round playable voice bubbles instead of file attachments.
Changes:
- Add asVoice option to TelegramSendOpts (defaults to true)
- When asVoice is true (default): use api.sendVoice() for voice bubbles
- When asVoice is false: use api.sendAudio() for traditional audio files
This gives callers control: voice messages for TTS/quick responses,
audio files for music/podcasts with metadata display.