Analyze Contacts

Validates the import template CSV (multipart/form-data, field 'file'; the template is the one the app downloads, header names are matched loosely and missing columns are allowed) without creating anything: one entry per data row with the subcontractor it would become, or a skip reason (no name, an invalid type, UTR, VAT rate, flag or verification, the CIS details the type requires missing). A file with no rows, or that is not CSV, is 400.