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

classes/app.go

@@ -0,0 +1,32 @@
+package classes
+
+import (
+	"git.golem15.com/golem15/sm-quizzes-plugin/graders"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/conga"
+	"gorm.io/gorm"
+)
+
+// ForApp builds the service the application runs on db: the scoring and
+// test grace from its config, the grader registry golem15.quizzes
+// published at boot, its event bus and its job queue. Each part that is
+// missing stays nil and the service works without it (no graders, no
+// events, no jobs). A nil app gives a service with the default grace only.
+func ForApp(app *backpack.App, db *gorm.DB) *Service {
+	if app == nil {
+		return &Service{DB: db, Grace: DefaultTestGrace}
+	}
+	s := &Service{
+		DB:      db,
+		Scoring: ScoringConfigFrom(app.Config),
+		Grace:   TestGraceFrom(app.Config),
+		Events:  app.Events,
+	}
+	if reg, ok := app.Lookup[*graders.Registry](); ok {
+		s.Graders = reg
+	}
+	if m, err := conga.From(app); err == nil && m != nil {
+		s.Jobs = m
+	}
+	return s
+}