Add daily usage metrics (CPU-minutes, RAM GB-minutes)

Introduce pre-computed daily usage rollups from sandbox_metrics_snapshots.
An hourly background worker aggregates completed days, while today's
usage is computed live from snapshots at query time for freshness.

Backend: new daily_usage table, rollup worker, UsageService, and
GET /v1/capsules/usage endpoint with date range filtering (up to 92 days).

Frontend: replace Usage page placeholder with bar charts (Chart.js),
summary total cards, and preset/custom date range controls.

pkg/cpserver/run.go

@@ -210,6 +210,10 @@ func Run(opts ...Option) {
 	sampler := api.NewMetricsSampler(queries, 10*time.Second)
 	sampler.Start(ctx)
 
+	// Start daily usage rollup (pre-computes CPU-minutes and RAM-MB-minutes).
+	rollup := api.NewDailyUsageRollup(queries, time.Hour)
+	rollup.Start(ctx)
+
 	// Start extension background workers.
 	for _, ext := range o.extensions {
 		for _, worker := range ext.BackgroundWorkers(sctx) {