WEBUSAGE-2b dashboard UI: the surface that replaces the manual SQL pull

Dashboard UI over the read-view schema (WEBUSAGE-5). Replaces the manual SQL pull as the thing a person actually opens: cross-app route ranking, per-app volume over time, and the arrival view that answers 'is this app still sending' - which today only rollup.sh's 24h fresh-arrival check can answer and only once a day. Scope: where it is hosted and behind what auth (the telemetry Postgres is standalone on venus, not Supabase, so this is NOT a Next.js/Supabase surface by default and that choice needs stating), what it queries (the read views only, never usageEvents directly), and the coverage limit printed on the page itself - raw rows purge at 90d, so any window longer than that is answering from usageDaily and must say so. Blocked on WEBUSAGE-5 for the query surface and WEBUSAGE-6 for who may see other apps' rows.