COUNT(*) OVER() forced SQLite to visit every matching file_group
before LIMIT, which is a full table scan of the (large) file_group
table — ~22s cold on a Pi for videos ordered by download date.
Count separately and cache for 60s by filters so indexed orders can
stop after one page. Past-the-end pages now report the true total.
Videos/archives/docs pagers keep totalPages across page changes.
