#1364 Metrics logs & live dashboard

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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.