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/lyra_worker/registry.py

@@ -13,6 +13,7 @@ from lyra_worker.browser import MbBrowser
 from lyra_worker.probe import AudioProbe
 from lyra_worker.resolver import MbResolver
 from lyra_worker.similarity._lastfm import LastfmSource
+from lyra_worker.similarity._lastfm_albums import LastfmAlbumPopularity
 from lyra_worker.similarity._listenbrainz import ListenBrainzSource
 from lyra_worker.similarity.base import SimilaritySource
 from lyra_worker.tagger import Tagger
@@ -67,3 +68,10 @@ def build_similarity_sources(config: dict, dsn: str | None = None) -> list[Simil
     if key:
         sources.append(LastfmSource(api_key=key, browser=build_browser(dsn)))
     return sources
+
+
+def build_album_popularity(config: dict, dsn: str | None = None):
+    """Last.fm album-popularity provider for discovery, or None when no Last.fm key is set
+    (discovery then surfaces newest albums only)."""
+    key = config.get("lastfm.api_key")
+    return LastfmAlbumPopularity(api_key=key) if key else None