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/openapi.yaml

@@ -613,6 +613,32 @@ paths:
                 items:
                   $ref: "#/components/schemas/Sandbox"
 
+  /v1/sandboxes/stats:
+    get:
+      summary: Get sandbox usage stats for your team
+      operationId: getSandboxStats
+      tags: [sandboxes]
+      security:
+        - apiKeyAuth: []
+      parameters:
+        - name: range
+          in: query
+          required: false
+          schema:
+            type: string
+            enum: [5m, 1h, 6h, 24h, 30d]
+            default: 1h
+          description: Time window for the time-series data.
+      responses:
+        "200":
+          description: Sandbox stats for the team
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/SandboxStats"
+        "400":
+          $ref: "#/components/responses/BadRequest"
+
   /v1/sandboxes/{id}:
     parameters:
       - name: id
@@ -1578,6 +1604,57 @@ components:
             after this duration of inactivity (no exec or ping). 0 means
             no auto-pause.
 
+    SandboxStats:
+      type: object
+      properties:
+        range:
+          type: string
+          enum: [5m, 1h, 6h, 24h, 30d]
+        current:
+          type: object
+          properties:
+            running_count:
+              type: integer
+            vcpus_reserved:
+              type: integer
+            memory_mb_reserved:
+              type: integer
+            sampled_at:
+              type: string
+              format: date-time
+              nullable: true
+        peaks:
+          type: object
+          description: Maximum values over the last 30 days.
+          properties:
+            running_count:
+              type: integer
+            vcpus:
+              type: integer
+            memory_mb:
+              type: integer
+        series:
+          type: object
+          description: Parallel arrays for chart rendering.
+          properties:
+            labels:
+              type: array
+              items:
+                type: string
+                format: date-time
+            running:
+              type: array
+              items:
+                type: integer
+            vcpus:
+              type: array
+              items:
+                type: integer
+            memory_mb:
+              type: array
+              items:
+                type: integer
+
     Sandbox:
       type: object
       properties: