feat: grade open answers through a grader registry in the background

- graders registry collected at boot from every HasGraders plugin,
  manual first; a duplicate or reserved code fails boot
- grading job on queue quizzes_grading with three attempts; a stale
  or reviewed answer is never touched; the last failure marks it failed
- learning answers enqueue at once, tests at close; a grade
  recalculates a finished attempt and fires AnswerGradedEvent after
  commit

events/events.go

@@ -0,0 +1,62 @@
+// Package events holds the plugin events of golem15.quizzes that other
+// plugins listen to with app.Events.Listen. Every event fires after the
+// write it announces has committed, so a listener never sees a score that
+// was rolled back; a listener error is logged and never undoes the write.
+// The package imports nothing from golem15.quizzes, so a listener can
+// depend on it alone.
+package events
+
+// Event names, for logs and documentation; the bus itself is keyed by type.
+const (
+	AnswerGradedEventName    = "quizzes.AnswerGraded"
+	AnswerReviewedEventName  = "quizzes.AnswerReviewed"
+	AttemptReviewedEventName = "quizzes.AttemptReviewed"
+)
+
+// AnswerGradedEvent is fired as *AnswerGradedEvent after a grader's score
+// of an open answer is stored (grading_status graded). The owner has not
+// confirmed it yet. MaxPoints is the question's points.
+type AnswerGradedEvent struct {
+	QuizID     uint
+	QuestionID uint
+	AttemptID  uint
+	AnswerID   uint
+	UserID     uint
+	Grader     string
+	Points     float64
+	MaxPoints  int
+	Feedback   string
+}
+
+// AnswerReviewedEvent is fired as *AnswerReviewedEvent after the owner
+// confirms or sets the score of an open answer (grading_status reviewed).
+// GraderPoints is the grader's score when there was one; Comment is the
+// owner's comment to the student. The reviewer is a frontend user
+// (ReviewerUserID, over MCP) or a backend administrator (ReviewerAdminID).
+type AnswerReviewedEvent struct {
+	QuizID          uint
+	QuestionID      uint
+	AttemptID       uint
+	AnswerID        uint
+	UserID          uint
+	Points          float64
+	MaxPoints       int
+	GraderPoints    *float64
+	Comment         *string
+	ReviewerUserID  *uint
+	ReviewerAdminID *uint
+}
+
+// AttemptReviewedEvent is fired as *AttemptReviewedEvent once every open
+// answer of a completed attempt is reviewed, with the attempt's final
+// score. MaxPoints is the attempt's stored maximum; Passed compares
+// ScorePercent with the quiz's passing score.
+type AttemptReviewedEvent struct {
+	QuizID       uint
+	AttemptID    uint
+	UserID       uint
+	ScorePoints  float64
+	MaxPoints    int
+	ScorePercent *float64
+	Passed       bool
+}