The /stats endpoint in this repo is wrong on edge cases and too slow — customers on big dashboards are timing out. It currently rescans and resorts the whole 300k-event log on every request, and the analytics team says the numbers don't match the documented semantics (nearest-rank percentiles, half-up two-decimal averages, null fields when nothing matches, proper 400s). Make it correct per the README and fast enough to meet the documented performance budget, without changing the API shape. `npm test` must stay green.