Turn an uploaded workbook into a dataset to build on

One uploaded .twb / .twbx / .pbix -> a dataset landed in this workspace, plus the intent that will build a dashboard on it. Deliberately stops short of generating: POST the returned intent and dataview_id to /v3/generate, which already streams progress and reports failures. data_source says where the rows came from and must be shown to the user — today it is always fitted_sample, rows synthesised to fit the workbook's own fields, because a third of Tableau uploads carry no data.