Start a CSV export of this company's Leads and return the export record. The export runs in the background: a BigQuery query job plus an extract job routinely takes longer than a request can wait. Poll GET /api/lead_export/ until it returns a 'url'. Rows are read from the BigQuery mirror of Spanner, which lags writes, so a Lead created moments ago may not be included — narrow with 'since' and treat freshness as best-effort. Columns are the standard Lead fields plus every property registered via /api/custom_property.