feat: YouTube music download system with admin dashboard
Sidecar architecture: edge functions (auth + DB) → youtube-worker container
(youtubei.js for playlist metadata, yt-dlp for audio downloads).
- Edge functions: youtube-playlist, youtube-process, youtube-status (CRUD)
- youtube-worker sidecar: Node.js + yt-dlp, downloads M4A to Supabase Storage
- Admin music page: import playlists, process queue, inline genre editing
- 12 music genres with per-track assignment and import-time defaults
- DB migrations: download_jobs, download_items tables with genre column
- Deploy script and CI workflow for edge functions + sidecar
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