Comprehensive design doc for the live DJ mixing feature:
- Dual-deck library mixing with crossfader (constant-power curve)
- External audio input: local sound card (ALSA/Pulse/JACK) and
network audio (Icecast source protocol for remote DJs)
- Session lifecycle with auto-playlist pause/resume and watchdog
- API endpoints, backend classes, frontend layout
- Phased implementation plan
- Open questions for team discussion