Schema: ALTER TABLE artists ADD COLUMN amazon_id TEXT with index, added via
_add_amazon_columns migration called after Discogs in _run_migrations.
SOURCE_ID_FIELD: add "amazon" -> "amazon_id" entry. find_library_artist_for_
source now looks up Amazon artists by slug before falling back to name match,
same as every other source. artist_source_detail already stamps artist_info
[source_id_field] = artist_id so the amazon_id is set on source-only payloads.
Tests: add "amazon": "amazon_id" to EXPECTED_SOURCE_ID_FIELD; revert test
assertion back to strict equality (SOURCE_ONLY_ARTIST_SOURCES == SOURCE_ID_
FIELD.keys() holds again now that amazon has a column).
Broque Thomas