admin/pools: compute per-pool traffic histograms lazily

[masterkain]

Parent: admin/pools async traffic follow-ups

Goal

Avoid building and shipping a traffic histogram for every Pool card when the chart is not being viewed

Acceptance criteria

• defer histogram computation until a Pool card is expanded or enters the viewport
• avoid sending unused chart payloads for off-screen and collapsed cards
• preserve the ApexTimeSeriesChart and phx-update="ignore" contracts
• cover card lifecycle, viewport transitions, and chart refresh behavior with focused tests
• verify the responsive Pool-card layouts after the loading behavior changes