Aggregated mode sent provider filters with no scan/date to /findings, which
the API rejects with 400, so requirement findings always rendered empty. Use
/findings/latest in aggregated mode (needs neither; scopes to the latest scan
per matching provider) and keep /findings for single-scan mode.
Also tidies the provider-filter helpers (review nits):
- Derive the filter keys/type from a FILTER_FIELD subset const
- Accept ReadonlyURLSearchParams so call sites drop the search-params wrap
Adds a regression test covering both aggregated and single-scan modes.