Read the page-view beacon: per-route LCP, INP, CLS, TTFB percentiles and daily unique visitors from page_vital #2776

Third part of #2674, after #2711. page_vital is written (one row per report, at most 20k a day, 90 days) and nothing reads it. Done: a page an admin can reach shows per route pattern the p50/p75/p95 of each metric and unique visitors per day, from one bounded query; tested.

Parked by the crew: the run ended without a pull: declined: the table this issue reads does not exist yet. page_vital, /_measure and the beacon script appear nowhere on main, in this branch, or in any remote branch. They belong to #2711, which is still o

Parked by the crew: the run ended without a pull: declined: nothing has changed since the first decline. origin/main is still at e9e61509, and page_vital, /_measure and the beacon script appear nowhere in the repository. They come from #2711, which is st

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the run ended without a pull: declined: the table this issue reads does not exist yet. page_vital, /_measure and the beacon script appear nowhere on main, in this branch, or in any remote branch. They belong to #2711, which is still o
2. the run ended without a pull: declined: nothing has changed since the first decline. origin/main is still at e9e61509, and page_vital, /_measure and the beacon script appear nowhere in the repository. They come from #2711, which is st

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

State: dropped. It repeats #2712 (visitors and page speed as a /boards source), which reads the same beacon data and correctly waits for #2711 to create it. Both runs declined because the table #2711 creates does not exist yet. #2712 already covers the per-route percentiles and daily visitors.