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

worker/lyra_worker/similarity/_lastfm_albums.py

@@ -0,0 +1,53 @@
+import requests
+
+from lyra_worker.apicache import cached_api
+
+_BASE = "https://ws.audioscrobbler.com/2.0/"
+
+
+def _parse_top_albums(data) -> list[list]:
+    """Parse a Last.fm artist.gettopalbums response into [[name, mbid|None, playcount], …]
+    ordered by playcount desc. A single-album body arrives as a dict; an error body → []."""
+    if isinstance(data.get("error"), (int, float)):
+        return []
+    rows = data.get("topalbums", {}).get("album", [])
+    if isinstance(rows, dict):
+        rows = [rows]
+    out: list[list] = []
+    for r in rows:
+        name = r.get("name") or ""
+        if not name:
+            continue
+        out.append([name, r.get("mbid") or None, int(r.get("playcount") or 0)])
+    out.sort(key=lambda a: a[2], reverse=True)
+    return out
+
+
+class LastfmAlbumPopularity:
+    """Ranks an artist's albums by Last.fm playcount, so discovery can surface each similar
+    artist's most-played album (alongside their newest). Read-through cached in ApiCache; any
+    failure yields [] so the sweep just falls back to newest-only."""
+
+    def __init__(self, api_key, base_url=_BASE, timeout: float = 15.0, limit: int = 25):
+        self._key = api_key
+        self._base = base_url
+        self._timeout = timeout
+        self._limit = limit
+
+    def _fetch(self, artist_name: str) -> list[list]:
+        try:
+            res = requests.get(self._base, params={
+                "method": "artist.gettopalbums", "artist": artist_name,
+                "api_key": self._key, "format": "json", "limit": str(self._limit),
+                "autocorrect": "1",
+            }, timeout=self._timeout)
+            return _parse_top_albums(res.json())
+        except Exception:  # a Last.fm outage must never abort the sweep
+            return []
+
+    def top_albums(self, conn, artist_name: str) -> list[list]:
+        norm = (artist_name or "").strip().lower()
+        if not norm:
+            return []
+        return cached_api(conn, f"lfm-artist-top-albums:{norm}", 43200,
+                          lambda: self._fetch(artist_name))

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