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.

models/question_hooks.go

@@ -0,0 +1,45 @@
+package models
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// wasOpenKey is the statement setting by which BeforeSave tells AfterSave
+// that the saved question was open before.
+const wasOpenKey = "golem15.quizzes:question_was_open"
+
+// BeforeSave notes whether a stored question that is saved as another
+// type was open, for AfterSave. Every write path (the admin form and
+// relation manager, the manage API and MCP, the importer) saves through
+// GORM, so the hooks cover them all.
+func (q *Question) BeforeSave(tx *gorm.DB) error {
+	if q.ID == 0 || q.Type == TypeOpen {
+		return nil
+	}
+	var stored string
+	err := tx.Session(&gorm.Session{NewDB: true}).Model(&Question{}).
+		Where("id = ?", q.ID).Limit(1).Pluck("type", &stored).Error
+	if err != nil {
+		return err
+	}
+	if stored == TypeOpen {
+		tx.InstanceSet(wasOpenKey, true)
+	}
+	return nil
+}
+
+// AfterSave ends the grading of the answers to a question that stopped
+// being open, in the same transaction as the save: their grading_status
+// becomes NULL, so they leave the review queue and needs_review, and a
+// grading job still queued for them does nothing. Their text, grades and
+// points stay for the record (stored scores are never touched).
+func (q *Question) AfterSave(tx *gorm.DB) error {
+	if was, ok := tx.InstanceGet(wasOpenKey); !ok || was != true {
+		return nil
+	}
+	return tx.Session(&gorm.Session{NewDB: true}).Model(&AttemptAnswer{}).
+		Where("question_id = ? AND grading_status IS NOT NULL", q.ID).
+		Updates(map[string]any{"grading_status": nil, "updated_at": time.Now().UTC()}).Error
+}