Check whether the next ended GL period can be closed

Read-only. Resolves the period named by periodId, or else the earliest open GL period that ended before today, and lists what blocks closing it: transactions in the period that still need review, open blocking threads, and unresolved blocking-severity close blockers. A period that has not ended reports only period_not_ended; an already-closed period reports ready: false with an already_closed blocker. When no period resolves, periodId is null and the only blocker is no_period. Open to every company member.