- 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
80ea79ee3e