fix(discover): drop suggestions for non-artists (no core discography)

ListenBrainz surfaces producers/session players/band members (Max Martin,
Dominic Howard) that co-occur in listens but have no releases of their own —
they showed as bare rows with an empty page. _derive_albums now drops a
surfaced artist (deletes their pending suggestion) when their MB discography
has no core release-group (Album/Single/EP, no secondary type), catching both
empty discographies and soundtrack/production-only credits, while keeping real
singles/EP-only artists. On browse failure the suggestion is kept.

Test fake browser gains default_core so scoring tests keep surfacing candidates.
worker 249 tests.

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

worker/lyra_worker/adapters/fakes.py

@@ -72,17 +72,24 @@ class FailingAdapter(_BaseFake):
 
 
 class FakeMbBrowser:
-    """In-memory MbBrowser for tests. `releases` maps artist mbid -> [ReleaseGroupInfo]."""
+    """In-memory MbBrowser for tests. `releases` maps artist mbid -> [ReleaseGroupInfo].
+    `default_core=True` returns a synthetic core Album for any artist not in `releases`, so
+    scoring tests don't trip the "no core discography → drop the suggestion" filter."""
 
-    def __init__(self, artists=None, releases=None):
+    def __init__(self, artists=None, releases=None, default_core=False):
         self._artists = list(artists or [])
         self._releases = dict(releases or {})
+        self._default_core = default_core
 
     def search_artist(self, name: str) -> list[ArtistHit]:
         return list(self._artists)
 
     def browse_release_groups(self, artist_mbid: str) -> list[ReleaseGroupInfo]:
-        return list(self._releases.get(artist_mbid, []))
+        if artist_mbid in self._releases:
+            return list(self._releases[artist_mbid])
+        if self._default_core:
+            return [ReleaseGroupInfo(f"rg-{artist_mbid}", "Album", "Album", (), "2000-01-01")]
+        return []
 
 
 class FakeAudioProbe: