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/db/models.go

@@ -41,6 +41,13 @@ type Channel struct {
 	UpdatedAt  pgtype.Timestamptz `json:"updated_at"`
 }
 
+type DailyUsage struct {
+	TeamID       pgtype.UUID    `json:"team_id"`
+	Day          pgtype.Date    `json:"day"`
+	CpuMinutes   pgtype.Numeric `json:"cpu_minutes"`
+	RamMbMinutes pgtype.Numeric `json:"ram_mb_minutes"`
+}
+
 type Host struct {
 	ID               pgtype.UUID        `json:"id"`
 	Type             string             `json:"type"`