feat(discover): Last.fm album-popularity helper

LastfmAlbumPopularity.top_albums(conn, name) ranks an artist's albums by
Last.fm playcount (read-through ApiCache, 12h; errors -> []). registry
build_album_popularity returns it only when lastfm.api_key is set.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_lastfm_albums.py

@@ -0,0 +1,29 @@
+from lyra_worker.registry import build_album_popularity
+from lyra_worker.similarity._lastfm_albums import _parse_top_albums
+
+
+def test_parse_orders_by_playcount_and_keeps_mbids():
+    data = {"topalbums": {"album": [
+        {"name": "Older Hit", "mbid": "rg-old", "playcount": "5000"},
+        {"name": "Newer", "mbid": "", "playcount": "9000"},
+        {"name": "No Plays", "playcount": "0"},
+    ]}}
+    assert _parse_top_albums(data) == [
+        ["Newer", None, 9000],   # highest playcount first; empty mbid -> None
+        ["Older Hit", "rg-old", 5000],
+        ["No Plays", None, 0],
+    ]
+
+
+def test_parse_wraps_a_single_album_dict():
+    data = {"topalbums": {"album": {"name": "Only One", "mbid": "rg1", "playcount": "3"}}}
+    assert _parse_top_albums(data) == [["Only One", "rg1", 3]]
+
+
+def test_parse_returns_empty_on_error_body():
+    assert _parse_top_albums({"error": 6, "message": "bad"}) == []
+
+
+def test_build_album_popularity_none_without_key():
+    assert build_album_popularity({}) is None
+    assert build_album_popularity({"lastfm.api_key": "abc"}) is not None