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/recommendation.sql.go

@@ -97,6 +97,83 @@ func (q *Queries) ListLastPlayedArtistsForUser(ctx context.Context, arg ListLast
 	return items, nil
 }
 
+const listMostPlayedTracksForArtist = `-- name: ListMostPlayedTracksForArtist :many
+WITH plays AS (
+    SELECT track_id, count(*) AS cnt
+    FROM play_events
+    WHERE user_id = $2 AND was_skipped = false
+    GROUP BY track_id
+)
+SELECT t.id, t.title, t.album_id, t.artist_id, t.track_number, t.disc_number, t.duration_ms, t.file_path, t.file_size, t.file_format, t.bitrate, t.mbid, t.genre, t.added_at, t.updated_at,
+       albums.title AS album_title,
+       artists.name AS artist_name
+FROM plays p
+JOIN tracks t   ON t.id = p.track_id
+JOIN albums     ON albums.id  = t.album_id
+JOIN artists    ON artists.id = t.artist_id
+WHERE t.artist_id = $1
+  AND NOT EXISTS (
+      SELECT 1 FROM lidarr_quarantine q
+      WHERE q.user_id = $2 AND q.track_id = t.id
+  )
+ORDER BY p.cnt DESC, t.id
+LIMIT $3
+`
+
+type ListMostPlayedTracksForArtistParams struct {
+	ArtistID    pgtype.UUID
+	UserID      pgtype.UUID
+	ResultLimit int32
+}
+
+type ListMostPlayedTracksForArtistRow struct {
+	Track      Track
+	AlbumTitle string
+	ArtistName string
+}
+
+// Top tracks for one artist by this user's completed-play count (skips
+// excluded, quarantine filtered). Same projection as
+// ListMostPlayedTracksForUser plus an artist_id filter, so the handler
+// reuses trackRefFrom(row.Track, row.AlbumTitle, row.ArtistName).
+func (q *Queries) ListMostPlayedTracksForArtist(ctx context.Context, arg ListMostPlayedTracksForArtistParams) ([]ListMostPlayedTracksForArtistRow, error) {
+	rows, err := q.db.Query(ctx, listMostPlayedTracksForArtist, arg.ArtistID, arg.UserID, arg.ResultLimit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListMostPlayedTracksForArtistRow
+	for rows.Next() {
+		var i ListMostPlayedTracksForArtistRow
+		if err := rows.Scan(
+			&i.Track.ID,
+			&i.Track.Title,
+			&i.Track.AlbumID,
+			&i.Track.ArtistID,
+			&i.Track.TrackNumber,
+			&i.Track.DiscNumber,
+			&i.Track.DurationMs,
+			&i.Track.FilePath,
+			&i.Track.FileSize,
+			&i.Track.FileFormat,
+			&i.Track.Bitrate,
+			&i.Track.Mbid,
+			&i.Track.Genre,
+			&i.Track.AddedAt,
+			&i.Track.UpdatedAt,
+			&i.AlbumTitle,
+			&i.ArtistName,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listMostPlayedTracksForUser = `-- name: ListMostPlayedTracksForUser :many
 WITH plays AS (
     SELECT track_id, count(*) AS cnt