Backend: new overviewMetrics store — a single-tier (2s x 360 = 12min)
in-memory ring fed by ServerService.RefreshStatus for exactly the eight
overview series (cpu/mem/swap/diskUsage/netUp/netDown/tcpCount/udpCount).
No persistence, no gob snapshot, no system-history page: the only surface
is GET /panel/api/server/history/:metric/:bucket restricted to those
keys, which seeds the frontend curves. ~46KB bounded by construction;
the ring starts empty on every panel start.
Frontend: restore useOverviewHistory + the VitalTile/ThroughputCard/
ConnectionsCard sparklines and the IndexPage wiring exactly as they
were before P7 removed them along with the history module.
Muse
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