Tool checkout utilization: GET /api/v1/tools/{id}/utilization

New endpoint that pairs CHECK_OUT events with the next CHECK_IN, sums durations (clamped to the window), and returns total checkout-ms + utilization percentage + top-N users by time. Window defaults to 30 days (1-365 valid); topN defaults to 3 (max 10, set to 0 to omit per-user breakdown). Open checkouts close at 'now' and the response flags openAtWindowEnd=true. Tools:read scope; 14 new tests cover edge cases (clamping, open checkouts, force-override consecutive checkouts, top-N sorting).