Initiate a multipart upload and sign its parts

Advanced / large-file path: parts are uploaded directly to S3, so the client needs S3 egress. Agents without it should use createVideo with url. For large files: split the file into parts_count parts (≤100; each part ≥5MB except the last). Returns key, upload_id and one pre-signed entry per part. PUT each part's bytes to its signed URL, capture the ETag response header from each PUT, then call completeMultipartUpload with key, upload_id and parts=[{part, etag}]. Finally call createVideo with s3_key=<key>. The bytes go straight to S3, not through this API.