Import Timesheets

Imports the selected rows of a timesheet CSV (multipart/form-data: the file as 'file' and 'selections', a JSON array of { rowId } from the analysis). The file is analysed again and each selected row creates a purchase invoice (firing invoice.created, as any create) and, when the row has a payment date, a payment for its full payment amount; one bad row never aborts the rest, and a selected row with a skip reason fails with that reason. A file with no rows, not CSV, or no selection is 400.