Flushes the queued batch-settlement items in a batch into a single aggregate on-chain settlement: it settles the aggregate amount, writes a parent batch ledger row (reference.kind = "batch") plus any fee row, and marks the items flushed. Use it to consolidate many verified micro-payments (such as a high-frequency data feed) into one transfer. If the aggregate settlement fails, the queued items are marked failed for audit and retry. Requires admin authorization via the signed Authorization header.