fix: end grading when a question stops being open

WR-06: a Question save hook clears grading_status on the answers of a
question that changed type away from open, in the save's transaction,
so they leave the review queue and needs_review. GradeAnswer ignores a
question that is no longer open, and a choice or self-check re-answer
clears the text, grade and review columns of its row.

classes/grading.go

@@ -187,9 +187,9 @@ func fireAfterCommit[T any](ctx context.Context, s *Service, tx *gorm.DB, name s
 // open and no row lock held, so a slow grader blocks no student.
 //
 // A missing answer, one that is no longer pending (graded, failed or
-// reviewed by the owner) and one whose grading_seq differs from seq (the
-// student wrote a newer text) are left alone, as is an answer whose quiz
-// has no registered grader. Each run counts an attempt for the current
+// reviewed by the owner), one whose grading_seq differs from seq (the
+// student wrote a newer text) and one whose question is no longer open are
+// left alone, as is an answer whose quiz has no registered grader. Each run counts an attempt for the current
 // text first. A grader error, or a score ClampOpenPoints refuses, is
 // returned for River to retry while fewer than GradeMaxAttempts attempts
 // were made; the last one marks the answer failed with the error and
@@ -215,6 +215,10 @@ func (s *Service) GradeAnswer(ctx context.Context, answerID uint, seq int) error
 		}
 		return err
 	}
+	if question.Type != models.TypeOpen {
+		// The question stopped being open after the job was queued.
+		return nil
+	}
 	var attempt models.Attempt
 	if err := db.Preload("Quiz").Where("id = ?", row.AttemptID).Take(&attempt).Error; err != nil {
 		if errors.Is(err, gorm.ErrRecordNotFound) {