Close a GL period

Generates the period's closing entry (sweeping profit and loss to Retained Earnings) and then marks the period closed. The closing entry posts before the status flip so it lands in the period being closed; a stale closing entry left from an earlier close is voided and replaced. A closed period is a reconciliation marker: it still accepts postings and voids, and only a filing lock blocks them (#2411). Requires company role admin or owner in a signed-in session; API keys are refused with 403.