Add live stats page with metrics sampling and route split

- New sandbox_metrics_snapshots table sampled every 10s (60-day retention)
- Background MetricsSampler goroutine wired into control plane startup
- GET /v1/sandboxes/stats?range=5m|1h|6h|24h|30d endpoint with adaptive
  polling intervals; reserved CPU/RAM uses ceil(paused/2) formula
- StatsPanel component: 4 stat cards + 2 Chart.js line charts (straight
  lines, integer y-axis for running count, dual-axis for CPU/RAM)
- Range filter persisted in URL query param; polls update data silently
  (no blink — loading state only shown on initial mount)
- Split /dashboard/capsules into /list and /stats sub-routes with shared
  layout; capsuleRunningCount store syncs badge across routes
- CreateCapsuleDialog extracted as reusable component

internal/db/models.go

@@ -99,6 +99,15 @@ type Sandbox struct {
 	TeamID       string             `json:"team_id"`
 }
 
+type SandboxMetricsSnapshot struct {
+	ID               int64              `json:"id"`
+	TeamID           string             `json:"team_id"`
+	SampledAt        pgtype.Timestamptz `json:"sampled_at"`
+	RunningCount     int32              `json:"running_count"`
+	VcpusReserved    int32              `json:"vcpus_reserved"`
+	MemoryMbReserved int32              `json:"memory_mb_reserved"`
+}
+
 type Team struct {
 	ID        string             `json:"id"`
 	Name      string             `json:"name"`