feat(recommendation): SuggestArtists service for M5c

Add per-user artist-suggestion service ranking out-of-library MBIDs by
signal x similarity. Single-CTE SQL collects user likes (5x weight) and
recency-decayed plays, joins against artist_similarity_unmatched, and
filters in-library candidates plus non-terminal lidarr_requests. The
service resolves top-3 attribution seeds to artist names in a batched
GetArtistsByIDs call so the UI can render "because you liked X" reasons.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/dbq/artists.sql.go

@@ -69,6 +69,39 @@ func (q *Queries) GetArtistByName(ctx context.Context, name string) (Artist, err
 	return i, err
 }
 
+const getArtistsByIDs = `-- name: GetArtistsByIDs :many
+SELECT id, name, sort_name, mbid, created_at, updated_at FROM artists WHERE id = ANY($1::uuid[])
+`
+
+// Batched lookup used by M5c suggestion attribution to resolve top-3
+// contributing seed UUIDs back to artist names in one round-trip.
+func (q *Queries) GetArtistsByIDs(ctx context.Context, dollar_1 []pgtype.UUID) ([]Artist, error) {
+	rows, err := q.db.Query(ctx, getArtistsByIDs, dollar_1)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []Artist
+	for rows.Next() {
+		var i Artist
+		if err := rows.Scan(
+			&i.ID,
+			&i.Name,
+			&i.SortName,
+			&i.Mbid,
+			&i.CreatedAt,
+			&i.UpdatedAt,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listArtists = `-- name: ListArtists :many
 SELECT id, name, sort_name, mbid, created_at, updated_at FROM artists ORDER BY sort_name
 `