Personalized artist suggestions on /discover (search-input-empty state).
On-demand SQL ranks out-of-library MBIDs from artist_similarity_unmatched
(new table, populated by extending the M4b similarity worker) by per-user
signal: likes weighted 5x plus recency-decayed plays (exp(-age_days/30)).
Top-12 with top-3 contributing seeds attributed per card. Reuses the M5a
DiscoverResultCard + POST /api/requests artist-add path.
