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

db/queries/metrics.sql

@@ -0,0 +1,37 @@
+-- name: InsertMetricsSnapshot :exec
+INSERT INTO sandbox_metrics_snapshots (team_id, running_count, vcpus_reserved, memory_mb_reserved)
+VALUES ($1, $2, $3, $4);
+
+-- name: GetCurrentMetrics :one
+SELECT running_count, vcpus_reserved, memory_mb_reserved, sampled_at
+FROM sandbox_metrics_snapshots
+WHERE team_id = $1
+ORDER BY sampled_at DESC
+LIMIT 1;
+
+-- name: GetPeakMetrics :one
+SELECT
+    COALESCE(MAX(running_count), 0)::INTEGER      AS peak_running_count,
+    COALESCE(MAX(vcpus_reserved), 0)::INTEGER     AS peak_vcpus,
+    COALESCE(MAX(memory_mb_reserved), 0)::INTEGER AS peak_memory_mb
+FROM sandbox_metrics_snapshots
+WHERE team_id = $1
+  AND sampled_at > NOW() - INTERVAL '30 days';
+
+-- name: PruneOldMetrics :exec
+DELETE FROM sandbox_metrics_snapshots
+WHERE sampled_at < NOW() - INTERVAL '60 days';
+
+-- name: SampleSandboxMetrics :many
+-- Aggregates per-team resource usage from the live sandboxes table.
+-- paused sandboxes count at 50% (ceil) for capacity reservation.
+SELECT
+    team_id,
+    (COUNT(*) FILTER (WHERE status IN ('running', 'starting')))::INTEGER                                              AS running_count,
+    (COALESCE(SUM(vcpus)    FILTER (WHERE status IN ('running', 'starting')), 0)
+     + CEIL(COALESCE(SUM(vcpus)    FILTER (WHERE status = 'paused'), 0)::NUMERIC / 2))::INTEGER                      AS vcpus_reserved,
+    (COALESCE(SUM(memory_mb) FILTER (WHERE status IN ('running', 'starting')), 0)
+     + CEIL(COALESCE(SUM(memory_mb) FILTER (WHERE status = 'paused'), 0)::NUMERIC / 2))::INTEGER                     AS memory_mb_reserved
+FROM sandboxes
+WHERE status IN ('running', 'starting', 'paused')
+GROUP BY team_id;