MusicBrainz: Re-enable real artist search (was returning empty)

`MusicBrainzSearchClient.search_artists` has been a `return []` stub
since the feature landed, with a comment claiming the MB tab 'doesn't
show artists.' That's why kettui saw a missing Artists section on the
search page — not a missing render, a hardcoded empty list.

Re-enable it properly:

- New `strict=False` parameter on `MusicBrainzClient.search_artist`
  sends a bare Lucene query instead of `artist:"..."`. MusicBrainz
  matches bare queries against alias+artist+sortname indexes together,
  which is the right behavior for user-facing fuzzy search (finds
  typos, aliases, sortname variants). `strict=True` remains the
  default for enrichment/AcoustID callers that want exact matches.

- Adapter filters results to `score >= 80`. MB assigns a 0-100 Lucene
  score on every hit; the true artist + close variants score 100,
  tribute bands and lookalikes typically land in the 40-65 range.
  The cutoff keeps "Metallica" (100) and drops "Black Metallica
  Tribute Band" (60) without hand-curated lists.

- Results returned as the same `Artist` dataclass used elsewhere in
  the search-tab adapter layer. `popularity` carries the MB score
  (0-100) so the frontend can sort/highlight top matches if desired.

core/musicbrainz_client.py

@@ -72,40 +72,57 @@ class MusicBrainzClient:
         logger.info(f"MusicBrainz client initialized with user agent: {self.user_agent}")
     
     @rate_limited
-    def search_artist(self, artist_name: str, limit: int = 10) -> List[Dict[str, Any]]:
+    def search_artist(self, artist_name: str, limit: int = 10, strict: bool = True) -> List[Dict[str, Any]]:
         """
-        Search for artists by name
-        
+        Search for artists by name.
+
         Args:
             artist_name: Name of the artist to search for
             limit: Maximum number of results to return
-            
+            strict: When True (default), builds a phrase-match query against
+                the `artist` field only — correct for enrichment flows that
+                already know the exact name. When False, sends a bare query
+                which MusicBrainz matches against the alias, artist, AND
+                sortname indexes — the right behavior for user-facing fuzzy
+                search (finds "Metallica" from typing "metalica", matches
+                aliased names, etc.).
+
         Returns:
-            List of artist results with id, name, score, etc.
+            List of artist results with id, name, score, etc. MusicBrainz
+            assigns each result a `score` 0-100; the list is pre-sorted
+            score-descending by the server.
         """
         try:
             # Escape quotes and backslashes for Lucene query
             safe_name = artist_name.replace('\\', '\\\\').replace('"', '\\"')
-            
+
+            if strict:
+                query = f'artist:"{safe_name}"'
+            else:
+                # Bare query hits alias/artist/sortname indexes — much better
+                # recall for user typing. Still Lucene-escaped via the API's
+                # query parser.
+                query = safe_name
+
             params = {
-                'query': f'artist:"{safe_name}"',
+                'query': query,
                 'fmt': 'json',
                 'limit': limit
             }
-            
+
             response = self.session.get(
                 f"{self.BASE_URL}/artist",
                 params=params,
                 timeout=10
             )
             response.raise_for_status()
-            
+
             data = response.json()
             artists = data.get('artists', [])
-            
+
             logger.debug(f"Found {len(artists)} artists for query: {artist_name}")
             return artists
-            
+
         except Exception as e:
             logger.error(f"Error searching for artist '{artist_name}': {e}")
             return []

core/musicbrainz_search.py

@@ -133,9 +133,57 @@ class MusicBrainzSearchClient:
         self._art_cache[release_mbid] = url
         return url
 
+    # Score threshold for user-facing search results. MusicBrainz returns a
+    # Lucene score 0-100 on every match; exact name/alias hits score 100,
+    # partial/typo matches trend lower, and tribute bands / random
+    # lookalikes score 40-65. 80 is the cutoff that keeps the true artist
+    # and close variants while dropping unrelated noise.
+    _MIN_SCORE = 80
+
     def search_artists(self, query: str, limit: int = 10) -> List[Artist]:
-        """MusicBrainz search tab doesn't show artists — only albums and tracks."""
-        return []
+        """Search MusicBrainz for artists by name.
+
+        Uses a bare Lucene query (no field prefix) so MusicBrainz searches
+        the alias, artist, AND sortname indexes together — much better
+        recall than strict `artist:"..."` phrase matching. Results are
+        filtered by score (>= 80) to drop tribute bands and unrelated
+        lookalikes.
+        """
+        try:
+            raw = self._client.search_artist(query, limit=limit, strict=False)
+            artists = []
+            for a in raw:
+                score = a.get('score', 0) or 0
+                if score < self._MIN_SCORE:
+                    continue
+
+                mbid = a.get('id', '')
+                name = a.get('name', '')
+                if not mbid or not name:
+                    continue
+
+                # Genres from MB tags (user-applied categorical labels). Each
+                # tag has {name, count}; keep the top-weighted ones.
+                tags = a.get('tags', []) or []
+                genres = [t.get('name') for t in tags if t.get('name')][:5]
+
+                external_urls = {
+                    'musicbrainz': f'https://musicbrainz.org/artist/{mbid}'
+                }
+
+                artists.append(Artist(
+                    id=mbid,
+                    name=name,
+                    popularity=score,  # Reuse score as popularity (0-100)
+                    genres=genres,
+                    followers=0,  # MusicBrainz doesn't track followers
+                    image_url=None,  # MB doesn't store artist images directly
+                    external_urls=external_urls,
+                ))
+            return artists
+        except Exception as e:
+            logger.warning(f"MusicBrainz artist search failed: {e}")
+            return []
 
     def search_albums(self, query: str, limit: int = 10) -> List[Album]:
         """Search MusicBrainz for releases (albums)."""