Scrapes Spotify's embed endpoint to extract track data from any public
playlist or album URL. Full discovery flow with Deezer parity: parse →
card → discovery modal → live progress → sync → download missing.
- New scraper: core/spotify_public_scraper.py (embed endpoint parsing)
- 12 API endpoints mirroring Deezer's discovery/sync/download flow
- WebSocket live discovery updates via spotify_public_discovery_states
- Green-branded tab, cards, and input styling (#1DB954)
- Album vs playlist detection with distinct card icons (💿/🎵)
- Download persistence: card click reopens download modal after close
- Card phase reset on cancel/completion (closeDownloadMissingModal)
- Backend download linking for spotify_public_ and deezer_ prefixes
- Completion handlers (V2 + no-missing-tracks) for both platforms
- Source URLs stored on mirrored playlists for future auto-refresh
