feat(worker): Last.fm artist.getSimilar as a second discovery SimilaritySource

Adds LastfmSource, resolving Last.fm's often-empty artist mbid via the
injected MB browser (name->mbid cached) so every emitted SimilarArtist.mbid
is a real MusicBrainz artist MBID. Registered in build_similarity_sources,
gated on lastfm.api_key so the ListenBrainz-only path is unchanged when
unset.

worker/lyra_worker/similarity/_lastfm.py

@@ -0,0 +1,69 @@
+import requests
+
+from lyra_worker.similarity.base import SimilarArtist
+
+_BASE = "https://ws.audioscrobbler.com/2.0/"
+
+
+class LastfmSource:
+    """SimilaritySource backed by Last.fm's artist.getsimilar.
+
+    Last.fm rows often carry an empty mbid, so names are resolved to a real
+    MusicBrainz artist MBID via the injected browser. Unresolvable rows are
+    skipped -- every emitted SimilarArtist.mbid must be a real MB artist MBID,
+    since downstream keys on it.
+    """
+
+    name = "lastfm"
+
+    def __init__(self, api_key, browser, base_url=_BASE, timeout: float = 15.0,
+                 similar_limit: int = 50):
+        self._key = api_key
+        self._browser = browser  # MbBrowser: .search_artist(name) -> list[ArtistHit]
+        self._base = base_url
+        self._timeout = timeout
+        self._limit = similar_limit
+        self._name_cache: dict[str, str | None] = {}
+
+    def _get(self, method: str, **params):
+        res = requests.get(
+            self._base,
+            params={"method": method, "api_key": self._key, "format": "json", **params},
+            timeout=self._timeout,
+        )
+        return res.json()
+
+    def health(self) -> bool:
+        try:
+            data = self._get("artist.getsimilar", artist="Radiohead", limit="1")
+            return not isinstance(data.get("error"), (int, float))
+        except requests.RequestException:
+            return False
+
+    def _resolve(self, name: str) -> str | None:
+        key = name.lower()
+        if key in self._name_cache:
+            return self._name_cache[key]
+        hits = self._browser.search_artist(name)
+        mbid = hits[0].mbid if hits else None
+        self._name_cache[key] = mbid
+        return mbid
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        data = self._get("artist.getsimilar", mbid=mbid, limit=str(self._limit))
+        if isinstance(data.get("error"), (int, float)):
+            return []
+        rows = data.get("similarartists", {}).get("artist", [])
+        if isinstance(rows, dict):
+            rows = [rows]
+        out: list[SimilarArtist] = []
+        for row in rows:
+            name = row.get("name") or ""
+            cand = row.get("mbid") or None
+            if not cand:
+                cand = self._resolve(name)
+            if not cand or cand == mbid or not name:
+                continue
+            score = float(row.get("match") or 0)
+            out.append(SimilarArtist(mbid=cand, name=name, score=score))
+        return out