How we compute per-URL TTFB

TTFB per URL is `quantileTDigest(0.50 / 0.75 / 0.95)` over `response_ms` in `log_hits`, over the requested window (default 30 days). Only hits with `response_ms > 0` are counted — some log formats omit the field.

Server-side TTFB from logs is a complement to the RUM CWV numbers — it shows the backend latency Googlebot experiences, before any client-side variability.