#1364 Metrics logs & live dashboard

描述

The Monitor tab only shows the last minute of values and keeps nothing once the page closes. With this change the
server writes every value the Monitor shows to a JSONL file once a second, whether or not a browser is open. A new
page, `/metrics-history`, draws that file as live line charts, so you can see how speed, VRAM, PCIe, power and the
other values moved during a task. No prompts or answers are written.