API method to create a batch from an explicit list of items and queue it. Replaces Classic's `Batch/Update`, the only route that accepted an arbitrary `Items[]` and which was not reachable from this API — every other batch endpoint here either acts on an existing batch by id or builds the item list itself. Pair it with the `cascade-items` endpoints on pages and content items to submit a user-pruned selection. An item list longer than one batch holds is split across several rather than rejected, so the caller does not have to page it themselves; every batchID comes back in the result. As with every batch endpoint, this only enqueues: Classic enforces the caller's permission on each item when it processes the batch, so an item the caller cannot act on fails within the batch while the rest proceed.