Close an assigned customer's GL period

Generates the period's closing entry and sweeps profit and loss to Retained Earnings, then marks the period closed. The closing entry posts before the status flip so it lands in the period being closed, and a re-close after a reopen writes a fresh entry rather than leaving the period closed without one (#2143). A closed period is a reconciliation marker and still accepts postings and voids (#2411). Caller must be an accountant assigned to the customer. The agent service API key may close a period as a high-impact operation (2026-10-06); other API keys cannot.