Closes the gap where similar artists only existed for WATCHLIST artists: a new
background worker populates them for the whole LIBRARY, slotting into the
existing enrichment-worker pattern (bubble + Manage Enrichment Workers modal,
status/pause/resume, matched/not_found/pending/errors).
Per source-matched library artist → get_musicmap_similar_artists(name, 25)
(the same matcher the artist-detail page uses: fetches MusicMap names, matches
each to the user's source chain — primary + active fallbacks — returns only
matched artists) → store via add_or_update_similar_artist keyed by the artist's
metadata source id, the SAME key the watchlist scanner + artist map use, so the
two cooperate (idempotent upsert + retry_days window).
- core/similar_artists_worker.py: pure seams (pick_source_artist_id,
map_payload_to_store_kwargs, process_artist) + the threaded worker; skips
artists not yet source-matched; classifies not_found vs transient error
(retry after 30d).
- DB migration: similar_artists_match_status / _last_attempted on artists
(mirrors every other source worker's tracking columns).
- Registered in EnrichmentService + instantiated in web_server, DEFAULT-PAUSED
(opt-in) like Amazon — MusicMap is scraped/outage-prone + this is library-wide.
- SERVICE_ENTITY_SUPPORT['similar_artists']=('artist',) so the modal breakdown
('artists with / without similars') + Retry work; manual-match (inapplicable
to a relationship) is gated out via relationship:true.
- 10 seam tests; existing 80 enrichment tests still pass.
Note: keys under profile 1 (single-profile setups); multi-profile is future work.
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