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/logger.go

@@ -0,0 +1,7 @@
+package classes
+
+import "git.golem15.com/golem15/summercms/modules/diary"
+
+// pluginLog is the plugin's logger: its records carry component=quizzes, so
+// config/log.yaml can raise or lower them (log.components.quizzes).
+var pluginLog = diary.For("quizzes")