Adds src/music/spotify/webapi.ts (client-credentials token, catalog
mappers, 429 retry) + tests, and threads the new "spotify" platform id
through the type unions in provider.ts and database.ts. Also widens the
downstream QueuedSong.platform union (audio/queue.ts) and the
getProviderFor parameter (bot/instance.ts) so tsc --noEmit stays clean;
these two are the necessary call-site fixes for the new union member.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>