Docs/GitHub webhook elevenlabs env

Summary

• document the GitHub App webhook URL and webhook secret setup in the README
• clarify where to configure GitHub App permissions and event subscriptions, including the minimum required permissions and optional ones for extra features
• add ELEVENLABS_API_KEY to turbo.json so builds receive the ElevenLabs env var

Why

The GitHub App setup was missing webhook guidance and left permission requirements implicit, which makes self-hosting setup more error-prone. Adding the ElevenLabs env var to Turbo keeps builds consistent with runtime configuration.