Update quiz question-bank assignments

Update up to BATCH_MAX_ITEMS quiz↔question-bank assignments in one request.

The quiz_id path resource is resolved once up front (document-level 404). Each item carries the question_bank_id natural key plus the writable fields; a bank not assigned to this quiz is a per-item not_found. An empty attributes set (only the key) is a no-op success. Within-batch duplicate detection is first-wins on question_bank_id (mirrors bind / unbind); subsequent duplicates are pre-marked duplicate_in_batch without reaching the service, so a logical update fires its QuestionBankAssignmentUpdated event exactly once. Response is 200 if all succeed, 207 if any fail, 400 if the envelope is empty or over the cap.

See api_v2/docs/batch-responses.md for the 200/207/400 status convention.

Required OAuth scope: quizzes:write