LIVE metric cards with trend

The five LIVE performance cards the portal shows above the session table — Live GMV, Live Units Sold, Live Count, Avg Engagement Rate and Avg Viewing Duration. Each card carries its value, unit, the percent change against the immediately preceding window of equal length, and chart_data: one point per day of the window, zero-filled on days with no sessions so the series length always matches the range.

Differs from POST /lives/summary — that returns flat window totals. This returns the same totals plus period-over-period change and a daily series, and accepts the portal's filters / search so the cards can be scoped to a creator or a GMV band. Engagement rate is a decimal (0.0-1.0), durations are seconds.

Single-shop only; defaults to the last 30 days ending today. LIVE analytics is region-gated — shops outside the supported regions get a 403 REGION_NOT_SUPPORTED. A shop with no sessions returns zeroed cards as a normal 200.