Adds get_recommendation_sources() — for each recommended similar artist it
resolves the polymorphic similar_artists.source_artist_id back to the display
names of the user's OWN artists (library + watchlist) that list it, by matching
against every provider-id column on both tables. The /api/discover/similar-artists
endpoint now attaches a 'because' array per recommendation so the UI can show
'because you have X, Y, Z' instead of just a count.
Seam tests cover: library + watchlist resolution across different provider-id
columns, dedup + name-sort, max_per cap, orphan source omission, profile scoping.
