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/api/metrics_sampler.go

@@ -0,0 +1,68 @@
+package api
+
+import (
+	"context"
+	"log/slog"
+	"time"
+
+	"git.omukk.dev/wrenn/sandbox/internal/db"
+)
+
+// MetricsSampler records per-team sandbox resource usage to
+// sandbox_metrics_snapshots every interval. It also prunes rows older than
+// 60 days on each tick to keep the table bounded.
+type MetricsSampler struct {
+	db       *db.Queries
+	interval time.Duration
+}
+
+// NewMetricsSampler creates a MetricsSampler.
+func NewMetricsSampler(queries *db.Queries, interval time.Duration) *MetricsSampler {
+	return &MetricsSampler{db: queries, interval: interval}
+}
+
+// Start runs the sampler loop until the context is cancelled.
+func (s *MetricsSampler) Start(ctx context.Context) {
+	go func() {
+		ticker := time.NewTicker(s.interval)
+		defer ticker.Stop()
+
+		// Sample immediately on startup.
+		s.run(ctx)
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				s.run(ctx)
+			}
+		}
+	}()
+}
+
+func (s *MetricsSampler) run(ctx context.Context) {
+	s.prune(ctx)
+	if err := s.sample(ctx); err != nil {
+		slog.Warn("metrics sampler: sample failed", "error", err)
+	}
+}
+
+func (s *MetricsSampler) sample(ctx context.Context) error {
+	rows, err := s.db.SampleSandboxMetrics(ctx)
+	if err != nil {
+		return err
+	}
+	for _, row := range rows {
+		if err := s.db.InsertMetricsSnapshot(ctx, db.InsertMetricsSnapshotParams(row)); err != nil {
+			slog.Warn("metrics sampler: insert snapshot failed", "team_id", row.TeamID, "error", err)
+		}
+	}
+	return nil
+}
+
+func (s *MetricsSampler) prune(ctx context.Context) {
+	if err := s.db.PruneOldMetrics(ctx); err != nil {
+		slog.Warn("metrics sampler: prune failed", "error", err)
+	}
+}