fix(db/m7-352): cast COALESCE to uuid so sqlc emits typed return

internal/db/queries/system_playlists.sql

@@ -86,6 +86,7 @@ SELECT t.id
 -- "Songs like X" seed selection. Returns the user's most-played non-skipped
 -- track by artist X in the last 7 days. Falls back to the artist's most-
 -- recent album's first track if no plays exist.
+-- Cast to uuid so sqlc infers pgtype.UUID rather than interface{}.
 SELECT COALESCE(
     (SELECT t.id
        FROM play_events pe
@@ -106,7 +107,7 @@ SELECT COALESCE(
                t.track_number NULLS LAST,
                t.id
       LIMIT 1)
-);
+)::uuid AS id;
 
 -- name: PickTopAlbumCoverForArtistByUser :one
 -- "Songs like X" cover. Most-played album by user; tie-break by most-recent