Upload several briefs

Upload several brief blobs (base64) in one request, in order. Each item gets the checks of POST /api/v1/storage/upload, all before anything is stored; the whole body must fit the 2 MB JSON limit. A server storing on 0G takes at most 4 items per request (400 BATCH_TOO_LARGE otherwise): its uploads run one at a time, and a larger batch would outlast a proxy timeout. All or nothing: a failed item fails the request with its index (error.details.index) and no root hash is returned; storage is content-addressed, so sending the batch again gets the same root hashes.