- Add global discovery_match_cache table to cache successful track matches
(title+artist+provider -> matched result) across all discovery sources
- Cache check before API search in YouTube, ListenBrainz, Tidal, and Beatport
discovery workers; cache write after high-confidence matches
- Re-discovering playlists or overlapping tracks across sources skips API lookups
- Fix Spotify tab sidebar forcing 2-column grid on mobile via inline JS styles
- Add mobile responsive styles for Spotify playlist cards (stack layout vertically)
