Show monthly usage when AI credits are unlimited

Currently when admins do not set any limit for AI credits, usage in status line shows **Monthly: no limit**.

It would be nice if showed the total usage for current month so one has a way to estimate their spending and act accordingly.

Start by locating the CLI status-line code that renders the monthly AI-credit limit and the code that obtains usage totals. Update the unlimited-credit case to show current-month usage while retaining the unlimited status, then verify the status line reports the total and remains correct when no limit is configured.

索引モデルが issue の本文から書いたものです。