Bulk Approve Transactions

Approve multiple pending transactions in one request. Body: {"transaction_ids": [uuid, ...]} Each id is processed independently through the same approval logic as the single-approve endpoint (maker-checker, guard, balance apply, audit, ledger, publish). Failures do NOT abort the batch. Idempotent: already- APPROVED ids return in the `approved` list. Already-declined ids return in `rejected` with the reason from the helper. Response: {"approved": [uuid, ...], "rejected": [{"id": uuid, "reason": str}, ...]}