Analyze Timesheets

Validates a filled-in timesheet CSV (multipart/form-data, field 'file'; header names are matched case-insensitively and missing columns are allowed) without creating anything: one entry per data row with the purchase invoice it would become, or a skip reason (unknown or non-subcontractor code, bad date or number, invalid VAT rate, a payment date in a tax month whose return is submitted). A file with no rows, or that is not CSV, is 400.