Function api_what_report 

pub fn api_what_report(
    corpus: &str,
    service: &str,
    severity: &str,
    category: &str,
    offset: Option<i64>,
    page_size: Option<i64>,
    pool: &State<DbPool>,
) -> Result<Json<WhatReportDto>, Status>

The what drill-down (agent twin of the category screen): one row per what within a category, descending by task count, paginated. 400 on an unknown severity, 404 on an unknown corpus/service.