Adds the approved design for a new optional `spotify` MusicProvider that
streams real Spotify audio via a librespot-family sidecar behind one
SpotifyAudioBackend interface (go-librespot on Linux/Docker, Rust librespot
on Windows), with metadata via the Spotify Web API. Disabled by default,
opt-in, Premium-only, ToS-risk warned. Includes verified research appendix.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
