Update question banks

Update up to BATCH_MAX_ITEMS question banks in a single request.

The data array must contain at least one item and at most BATCH_MAX_ITEMS items. Each item must carry the obfuscated question-bank id and an attributes object with the fields to update. A failure on one row does not abort the rest. Response is 200 if all items succeed, 207 if any fail, 400 if the request envelope is empty or over the cap.

See api_v2/docs/batch-responses.md for the 200/207/400 status convention and per-item error code vocabulary.

Required OAuth scope: question-banks:write