feat(server): similar-artists + per-user artist top-tracks endpoints

GET /api/artists/{id}/similar — in-library artists ranked by similarity
score (deduped across sources), ArtistRef list with cover + album count.
GET /api/artists/{id}/top-tracks — current user's most-played tracks for
the artist (skips excluded, quarantine filtered).

internal/db/dbq/similarity.sql.go

@@ -154,6 +154,77 @@ func (q *Queries) ListPlayedTracksNeedingSimilarity(ctx context.Context, limit i
 	return items, nil
 }
 
+const listSimilarArtistsForArtist = `-- name: ListSimilarArtistsForArtist :many
+SELECT artists.id, artists.name, artists.sort_name, artists.mbid, artists.created_at, artists.updated_at, artists.artist_thumb_path, artists.artist_fanart_path, artists.artist_art_source, artists.artist_art_sources_version,
+       cov.id AS cover_album_id,
+       cnt.album_count::bigint AS album_count
+FROM (
+    SELECT artist_b_id, max(score) AS sim_score
+    FROM artist_similarity
+    WHERE artist_a_id = $1
+    GROUP BY artist_b_id
+) s
+JOIN artists ON artists.id = s.artist_b_id
+LEFT JOIN LATERAL (
+    SELECT id FROM albums
+    WHERE artist_id = artists.id AND cover_art_path IS NOT NULL
+    ORDER BY created_at DESC LIMIT 1
+) cov ON true
+LEFT JOIN LATERAL (
+    SELECT count(*) AS album_count
+    FROM albums WHERE artist_id = artists.id
+) cnt ON true
+ORDER BY s.sim_score DESC, artists.sort_name
+LIMIT $2
+`
+
+type ListSimilarArtistsForArtistParams struct {
+	SeedArtistID pgtype.UUID
+	ResultLimit  int32
+}
+
+type ListSimilarArtistsForArtistRow struct {
+	Artist       Artist
+	CoverAlbumID pgtype.UUID
+	AlbumCount   int64
+}
+
+// In-library artists similar to a seed artist, ranked by best similarity
+// score across sources (deduped per candidate). cover_album_id + album_count
+// mirror ListArtistsAlphaWithCovers so the strip renders identical cards.
+func (q *Queries) ListSimilarArtistsForArtist(ctx context.Context, arg ListSimilarArtistsForArtistParams) ([]ListSimilarArtistsForArtistRow, error) {
+	rows, err := q.db.Query(ctx, listSimilarArtistsForArtist, arg.SeedArtistID, arg.ResultLimit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListSimilarArtistsForArtistRow
+	for rows.Next() {
+		var i ListSimilarArtistsForArtistRow
+		if err := rows.Scan(
+			&i.Artist.ID,
+			&i.Artist.Name,
+			&i.Artist.SortName,
+			&i.Artist.Mbid,
+			&i.Artist.CreatedAt,
+			&i.Artist.UpdatedAt,
+			&i.Artist.ArtistThumbPath,
+			&i.Artist.ArtistFanartPath,
+			&i.Artist.ArtistArtSource,
+			&i.Artist.ArtistArtSourcesVersion,
+			&i.CoverAlbumID,
+			&i.AlbumCount,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const upsertArtistSimilarity = `-- name: UpsertArtistSimilarity :exec
 INSERT INTO artist_similarity (artist_a_id, artist_b_id, score, source, fetched_at)
 VALUES ($1, $2, $3, 'listenbrainz', now())