Coverage for fix 1.1:
TestBackfillMigration verifies the one-shot migration sets
match_status='matched' for rows that already have a populated
external ID (lastfm_url, musicbrainz_release_id,
musicbrainz_recording_id, tidal_id, qobuz_id) but NULL match_status,
and leaves rows without an ID untouched.
TestGetExistingIdColumnMapping verifies lastfm_worker reads
lastfm_url for all entity types and musicbrainz_worker reads the
correct per-type column (musicbrainz_id / musicbrainz_release_id /
musicbrainz_recording_id).
TestLastFMWorkerMarksMatched / TestTidalWorkerMarksMatched /
TestQobuzWorkerMarksMatched / TestMusicBrainzWorkerMarksMatched
verify each worker's _process_* short-circuit path sets
match_status='matched' (and does not re-call the external API) when
the entity already has an ID populated.
