Add per-metric stats and trend endpoints to vclMultiVertical.js. Refactor
CCPMetricsPage to use a unified MetricSelector that drives StatsBar, TrendChart,
DonutChart, and ForecastBurndownChart for the selected metric only. Remove the
separate Per-Metric Forecast Burndown section (now integrated). Fix trend query
double-counting when multiple uploads exist per vertical per month.
Closes#25