feat: add open questions with a pending grading state

- migration: open question type, rubric, max_length, quiz grader and
  the grading and review columns of attempt answers
- open answers are checked, stored as pending and never scored on
  answer; a changed text resets the grading state
- student JSON gains text, grading_status, grader_feedback,
  review_comment, max_length, needs_review and open_answers; the
  rubric is never serialized

models/question.go

@@ -9,13 +9,25 @@ const (
 	TypeSingle    = "single"
 	TypeMultiple  = "multiple"
 	TypeSelfCheck = "self_check"
+	TypeOpen      = "open"
 )
 
 // QuestionTypes lists the valid values of Question.Type.
-var QuestionTypes = []string{TypeSingle, TypeMultiple, TypeSelfCheck}
+var QuestionTypes = []string{TypeSingle, TypeMultiple, TypeSelfCheck, TypeOpen}
+
+// Limits of the answer length of an open question, in characters.
+const (
+	// DefaultMaxLength applies when MaxLength is nil.
+	DefaultMaxLength = 2000
+	// MaxMaxLength is the largest MaxLength the column accepts.
+	MaxMaxLength = 20000
+)
 
 // Question is one question of a quiz. A self_check question is a flashcard:
-// the user reveals AnswerText and marks whether they knew it.
+// the user reveals AnswerText and marks whether they knew it. An open
+// question is answered in free text and graded against AnswerText (the
+// model answer) and the owner-only Rubric; MaxLength caps the answer
+// (DefaultMaxLength when nil).
 type Question struct {
 	ID          uint      `gorm:"column:id;primaryKey"`
 	QuizID      uint      `gorm:"column:quiz_id"`
@@ -24,6 +36,8 @@ type Question struct {
 	Category    *string   `gorm:"column:category"`
 	AnswerText  *string   `gorm:"column:answer_text"`
 	Explanation *string   `gorm:"column:explanation"`
+	Rubric      *string   `gorm:"column:rubric"`
+	MaxLength   *int      `gorm:"column:max_length"`
 	SortOrder   int       `gorm:"column:sort_order"`
 	Points      int       `gorm:"column:points"`
 	CreatedAt   time.Time `gorm:"column:created_at"`
@@ -37,7 +51,7 @@ func (Question) TableName() string { return "golem15_quizzes_questions" }
 
 // Fillable lists the columns mass assignment may set.
 func (Question) Fillable() []string {
-	return []string{"quiz_id", "text", "type", "category", "answer_text", "explanation", "sort_order", "points"}
+	return []string{"quiz_id", "text", "type", "category", "answer_text", "explanation", "sort_order", "points", "rubric", "max_length"}
 }
 
 // Rules are checked by the admin API on every save.
@@ -45,9 +59,10 @@ func (Question) Rules() map[string]string {
 	return map[string]string{
 		"quiz_id":    "required|integer|min:1",
 		"text":       "required",
-		"type":       "required|in:single,multiple,self_check",
+		"type":       "required|in:single,multiple,self_check,open",
 		"category":   "nullable|max:255",
 		"sort_order": "nullable|integer",
 		"points":     "required|integer|min:0",
+		"max_length": "nullable|integer|between:1,20000",
 	}
 }