Delete question banks

Delete up to BATCH_MAX_ITEMS question banks in a single request (CASCADE soft-delete).

Each item is a resource-identifier object ({type, id}). Per item the bank is resolved org-scoped + live (already-deleted / wrong-org / malformed ids surface as per-item not_found) and, in that item's own atomic block, the bank's questions and their answers are SOFT-deleted, every QuestionBankAssignment referencing the bank is HARD-removed (unbinding any quizzes that used it), and finally the bank itself is SOFT-deleted. The quizzes that referenced the bank are left ALIVE — only their assignment rows are removed.

Within-batch duplicate detection is first-wins on id; each deleted item echoes the caller's input id. Response is 200 if all deleted, 207 if any failed, 400 if the envelope is empty or over the cap. See api_v2/docs/batch-responses.md for the envelope shape.

Required OAuth scope: question-banks:write