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/migrations/20260325074949_metrics_snapshots.sql

@@ -0,0 +1,18 @@
+-- +goose Up
+
+CREATE TABLE sandbox_metrics_snapshots (
+    id                 BIGSERIAL    PRIMARY KEY,
+    team_id            TEXT         NOT NULL,
+    sampled_at         TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+    running_count      INTEGER      NOT NULL,
+    vcpus_reserved     INTEGER      NOT NULL,
+    memory_mb_reserved INTEGER      NOT NULL
+);
+
+-- All queries filter on team_id first then range-scan sampled_at.
+CREATE INDEX idx_metrics_snapshots_team_time
+    ON sandbox_metrics_snapshots (team_id, sampled_at DESC);
+
+-- +goose Down
+
+DROP TABLE sandbox_metrics_snapshots;