Fix Amazon artist detail: library upgrade lookup and artist images

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.

core/amazon_client.py

@@ -601,6 +601,25 @@ class AmazonClient:
         """Not available from Amazon Music — returns None for compatibility."""
         return None
 
+    def _get_artist_image_from_albums(self, artist_id: str) -> Optional[str]:
+        """Return an album cover as artist image stand-in (T2Tunes has no artist images)."""
+        search_name = _unslugify(artist_id)
+        try:
+            items = self.search_raw(search_name, types="track,album")
+        except AmazonClientError:
+            return None
+        name_lower = search_name.lower()
+        for item in items:
+            if _primary_artist(item.artist_name).lower() != name_lower:
+                continue
+            asin = item.album_asin or item.asin
+            if not asin:
+                continue
+            metas = self._fetch_album_metas([asin])
+            if asin in metas and metas[asin].get("image"):
+                return metas[asin]["image"]
+        return None
+
     # ==================== Interface Aliases (match DeezerClient method names) ====================
     get_album_metadata = get_album
     get_artist_info = get_artist

core/artist_source_lookup.py

@@ -58,19 +58,18 @@ def find_library_artist_for_source(
     Returns ``None`` on miss or on any database error.
     """
     column = SOURCE_ID_FIELD.get(source)
-    if not column:
-        return None
 
     try:
         with database._get_connection() as conn:
             cursor = conn.cursor()
-            cursor.execute(
-                f"SELECT id, name FROM artists WHERE {column} = ? LIMIT 1",
-                (str(source_artist_id),),
-            )
-            row = cursor.fetchone()
-            if row:
-                return row[0]
+            if column:
+                cursor.execute(
+                    f"SELECT id, name FROM artists WHERE {column} = ? LIMIT 1",
+                    (str(source_artist_id),),
+                )
+                row = cursor.fetchone()
+                if row:
+                    return row[0]
 
             if artist_name and active_server:
                 cursor.execute(

tests/metadata/test_artist_source_lookup.py

@@ -68,11 +68,11 @@ class TestSourceIdFieldMapping:
             "(and the test body) to match."
         )
 
-    def test_source_only_set_matches_mapping_keys(self):
-        """Sources eligible for the source-only fallback must all have a
-        column to look them up by — otherwise the upgrade path silently
-        returns None."""
-        assert SOURCE_ONLY_ARTIST_SOURCES == frozenset(SOURCE_ID_FIELD.keys())
+    def test_source_only_set_contains_all_mapped_sources(self):
+        """Every source with a SOURCE_ID_FIELD column must also be in
+        SOURCE_ONLY_ARTIST_SOURCES. The reverse is not required — sources
+        without a DB column (e.g. amazon) use name-based lookup only."""
+        assert frozenset(SOURCE_ID_FIELD.keys()).issubset(SOURCE_ONLY_ARTIST_SOURCES)
 
     def test_every_mapped_column_exists_on_artists_table(self, db):
         """Regression for the 2026-04 ``deezer_artist_id`` typo: every column