feat: polling API, HMAC webhook signing, session-only keys, prod audio fix
- Add polling API (GET/POST /api/fights/poll) so bots don't need public URLs
- Add HMAC-SHA256 webhook signing (X-Botfights-Signature header)
- Stop auto-persisting nsec keys — session-only by default with opt-in "Remember on this device"
- Fix production TTS: add wav/mp3/ogg MIME types, /audio/* route, SPA blocklist
- Overhaul docs: mode selector (poll vs webhook), AI-first bot examples, security tab
- Fix duplicate sign-in buttons, login flow bugs
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>