Create up to 100 cards in one transaction

All-or-nothing batch create - one invalid item rolls back the whole batch and the error names the offending index (cards[3]: …). Cards without an explicit position append to the end of their column in request order. Unlike the single-card create, the assignee is NOT defaulted to the caller. Bulk requests never send notification emails; activity events and SSE broadcasts fire normally. Body limit 1 MiB.