Bind question banks to a quiz

Bind up to BATCH_MAX_ITEMS question banks to a quiz in a single request.

The quiz_id path resource is resolved once up front — a malformed / cross-org / soft-deleted quiz is a document-level 404. Within-batch duplicate detection is first-wins on question_bank_id (mirrors unbind / delete_groups); subsequent duplicates are pre-marked duplicate_in_batch without reaching the service, so a logical bind fires its QuestionBankAssignmentBound event exactly once. Each surviving item is processed independently in its own atomic block: a bad question_bank_id is a per-item not_found; re-binding an already-bound bank UPDATES the row (update_or_create on the natural key, no IntegrityError). 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