WR-03: every new or changed open answer in a learning quiz with a
grader is a paid grading call. Add per-user buckets golem15.quizzes.answer
(60/min) and golem15.quizzes.start (10/min), configurable under
golem15.quizzes.throttle.*.
- Service.Start resumes an open test attempt, closes expired ones at
their deadline and refuses a start past max_attempts under an
advisory lock; StartAttempt stays as a wrapper
- answers to a test are accepted until the deadline plus
golem15.quizzes.test.grace_seconds; one_way answers are final
- a test covers the whole quiz and keeps a stable per-attempt order
- 409 bodies carry a code; list and attempt rows carry attempts_left,
question_types and seconds_left; import accepts the test keys
