37725457 fixed _match_to_itunes to use the real iTunes client and flagged
the cross-source corruption as a possibility. Boulder's live DB proves it
happened: 6 of his 9 watchlist "iTunes" ids EQUAL the artist's Deezer id
(Taylor Swift's "iTunes" id was her Deezer id 12246; the real one is
159260351) — written back when the misnamed MetadataService.itunes slot
held a DeezerClient. The June-4 batch (Green Day, SOAD, Vulfpeck, ...) got
NULL instead because the slot now holds the Spotify primary.
The fix alone can't heal those rows: the backfill only fills EMPTY ids, so
a wrong non-empty id is permanent. New migration clears itunes_artist_id
where it equals deezer_artist_id (the corruption signature — distinct id
spaces, so a legitimate equal pair is effectively impossible, and the worst
case is a NULL that re-matches correctly on the next scan). Idempotent by
construction; similar_artists checked clean (its backfill always used the
registry correctly).
Tests: corrupted row cleared / legit + no-deezer rows kept / idempotent —
via a real re-init with the per-process init memo cleared (an app restart).
58df4632c4