Unbind question banks from a quiz

Unbind (HARD-delete) up to BATCH_MAX_ITEMS assignments from a quiz.

The quiz_id path resource is resolved once up front (document-level 404). Within-batch duplicate detection is first-wins on question_bank_id (mirrors delete_groups); subsequent duplicates are pre-marked duplicate_in_batch without reaching the service. Each surviving item is processed in its own atomic block — a bank not assigned to this quiz is a per-item not_found. QuestionBankAssignment is not a SoftDeletionModel, so the rows are permanently removed. 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 envelope shape.

Required OAuth scope: quizzes:write