Chat figures that cannot be derived from a page

Counts computed with COUNT(*) over the caller’s full scope, for the dashboard.

Exists because the figure it replaces was summed from the thread list, which is bounded at 500 — a total assembled from a page reports truncation as completeness (see issues #180, #184, #188, #197).

Scope matches GET /chat/threads: an admin counts every thread, a user counts only threads they are still a participant in.

“Today” is UTC midnight to now. The boundary is fixed server-side and is NOT accepted from the caller.