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

controllers/api/handlers.go

@@ -259,13 +259,9 @@ func (h *Handlers) service(w http.ResponseWriter) (*classes.Service, bool) {
 	if h.Service != nil {
 		return h.Service(gdb), true
 	}
-	var scoring classes.ScoringConfig
-	grace := classes.DefaultTestGrace
-	if h.App != nil {
-		scoring = classes.ScoringConfigFrom(h.App.Config)
-		grace = classes.TestGraceFrom(h.App.Config)
-	}
-	return &classes.Service{DB: gdb, Scoring: scoring, Grace: grace}, true
+	// The scoring and grace come from the config as before; ForApp adds
+	// the grader registry, the job queue and the event bus.
+	return classes.ForApp(h.App, gdb), true
 }
 
 // input decodes a JSON body (an empty body is {}) and validates it. It