Starts a bulk invoice import. The request is multipart/form-data with a single required file part carrying the CSV (max 5 MiB, up to 1000 rows), e.g. curl -F "[email protected]". The response carries the new batch plus the detected column headers for the mapping step and a duplicate-upload warning. (The file part is described here rather than as a schema: the swag pipeline cannot attach the §10-required example to a binary multipart property.)