Accepts multipart/form-data with a file field named 'file'. Header: email,department_path[,is_primary] (org_unit_path is accepted as a silent back-compat alias for department_path). department_path is slash-separated unit names from root (e.g. Engineering/Platform/Payments); missing units are created. Emails resolve to directory people (created when missing, source=csv); no operational user is ever created. dryRun=true runs full validation with zero writes.