Part A of the deferred unification cleanup. The standalone artist-
detail endpoint used to 404 whenever `artist_id` wasn't a local library
primary key, which is exactly what source artists (Deezer/Spotify/
iTunes/etc.) have. That forced the Phase 4a revert: source artists had
to use the inline Artists page because this endpoint couldn't handle
them.
New behaviour:
- Library PK path — unchanged. Existing callers see the same response.
- `/api/artist-detail/<id>?source=<src>&name=<name>` with source in
(spotify, itunes, deezer, discogs, hydrabase, musicbrainz) — when
the library DB lookup misses, synthesize a response by:
• fetching artist image via metadata_service.get_artist_image_url
with source_override (the helper already backing /api/artist/
<id>/image)
• fetching discography via metadata_service.get_artist_detail_
discography with MetadataLookupOptions(source_override=source,
artist_source_ids={source: artist_id})
• returning { success, artist: {id, name, image_url, server_source:
null, genres: []}, discography, enrichment_coverage: {} }
- Library PK missing AND no source — preserves the 404 (caller didn't
give enough info to fall back).
Frontend plumbing: library.js loadArtistDetailData now appends
?source=<src>&name=<name> to the fetch URL when
artistDetailPageState.currentArtistSource is set. The field is already
seeded by navigateToArtistDetail's third arg (added during the earlier
unification work), so no new state plumbing is needed.
populateArtistDetailPage gracefully handles the missing-library-data
case per earlier exploration — owned_releases empty is fine,
enrichment_coverage optional, spotify_artist_id optional.
Part B will re-route the source-artist callsites (Search / Discover /
Watchlist / etc.) back through navigateToArtistDetail so they actually
exercise this new fallback path.
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