A repository a listing's search returned nothing for was read on its own on almost every
refresh, at four points each, and every finished turn moved the global listings epoch, so an
open page re-read everything after every turn in any project.

The silent repositories are now checked with one aliased search-coverage request (one point
for all of them), and a covered answer is remembered like a returned row. Turns move the
listings at most once per 30-second list cache window; the project's own pull requests still
refresh at once.

Fixes #19363

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>