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

classes/scoring.go

@@ -152,6 +152,27 @@ func Totals(points []float64, maxPoints int) (score, percent float64) {
 	return score, round(score / float64(maxPoints) * 100)
 }
 
+// ClampOpenPoints keeps a grader's score of an open question in
+// 0..points (a negative points value counts as 0), rounded to the nearest
+// 0.5 (D-02). NaN and ±Inf are not a score: it returns 0, false.
+func ClampOpenPoints(p float64, points int) (float64, bool) {
+	if math.IsNaN(p) || math.IsInf(p, 0) {
+		return 0, false
+	}
+	full := float64(max(points, 0))
+	p = math.Round(math.Min(math.Max(p, 0), full)*2) / 2
+	if p == 0 {
+		return 0, true // no negative zero in JSON
+	}
+	return p, true
+}
+
+// OpenCorrect reports whether an open answer scored p earns full marks: a
+// question with points earns is_correct only with all of them (D-02).
+func OpenCorrect(p float64, points int) bool {
+	return points > 0 && p == float64(points)
+}
+
 // round keeps two decimals.
 func round(f float64) float64 {
 	r := math.Round(f*100) / 100