/system updates over one server-sent-events stream and never reloads or polls #2720

First part of #2676.

Done:

• [ ] /system opens one EventSource per tab (src/http/sse.ts, src/web/live.ts) and patches samples in place; no meta refresh, no timer reload, no fetch on an interval
• [ ] the stream respects backpressure (a slow client drops samples, never buffers without bound) and resumes after a roll
• [ ] the first paint is server-rendered with the latest sample
• [ ] tests: /system's HTML has no refresh or interval reload; a sample written to the sampler arrives on the stream