Library upgrade: find_library_artist_for_source returned None immediately for
Amazon because SOURCE_ID_FIELD has no 'amazon' entry (no DB column for Amazon
artist IDs). The name-based fallback was unreachable. Fix: only skip the column
query when column is None, not the whole function — name lookup now runs for
any source when artist_name + active_server are provided.
Artist images: add AmazonClient._get_artist_image_from_albums so the standard
_get_artist_image_from_source path in metadata/artist_image.py can call it as
a fallback (same hook iTunes/Deezer/Discogs expose). Searches by unslugified
artist name, matches primary artist, fetches album cover from album_metadata.
Test: updated test_source_only_set_matches_mapping_keys → _contains_all_mapped_
sources to assert subset (not equality) — SOURCE_ONLY_ARTIST_SOURCES intentionally
includes sources without a DB column that rely on name-only lookup.
265fe5233e