Adds /api/system/stream — a Server-Sent Events feed driven by a single
per-process ticker that reads /proc directly and splices in the latest
host-side metrics.json each second. Subscribers share the connection so
N open tabs cost one ticker, and the ticker pauses entirely when nobody
is listening.
Frontend gets a singleton LiveSystem EventSource manager with auto-
reconnect, Page-Visibility integration (closes on tab hide), and last-
sample replay for late subscribers. Admin -> System gauges and the
dashboard memory + disk tile now tick at 1 Hz; trend charts and the
per-app table keep their 30 s poll because the underlying files only
regenerate once a minute.
Also adds /api/system/history as a thin range-query wrapper over the
existing 24 h JSON ring buffer — the binary ring backend will slot in
behind it in the next phase without changing the response shape.
Signed-off-by: librelad <librelad@digitalangels.vip>
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