feat: show node connection status and traffic stats in admin panel

Enable sing-box Clash/V2Ray APIs for per-user metrics, persist cumulative
traffic in SQLite, and refresh the dashboard every five seconds.

Co-authored-by: Cursor <cursoragent@cursor.com>

panel/templates/dashboard.html

@@ -18,14 +18,60 @@
     <button id="addBtn" class="btn primary">+ 添加节点</button>
   </section>
 
+  <section id="summaryBar" class="summary-bar">
+    <div class="summary-item">
+      <span class="summary-label">在线节点</span>
+      <strong id="summaryOnline">-</strong>
+    </div>
+    <div class="summary-item">
+      <span class="summary-label">实时上行</span>
+      <strong id="summaryUp">-</strong>
+    </div>
+    <div class="summary-item">
+      <span class="summary-label">实时下行</span>
+      <strong id="summaryDown">-</strong>
+    </div>
+    <div class="summary-item">
+      <span class="summary-label">统计服务</span>
+      <strong id="summaryStatus" class="status-text">检测中</strong>
+    </div>
+  </section>
+
   <div id="toast" class="toast hidden"></div>
   <div id="nodeList" class="node-list">
     {% for node in nodes %}
     <article class="node-card" data-id="{{ node.id }}">
       <div class="node-head">
-        <h2>{{ node.name }}</h2>
+        <div class="node-title">
+          <h2>{{ node.name }}</h2>
+          <span class="status-badge offline" data-role="status">检测中</span>
+        </div>
         <span class="tag">{{ node.created_at[:10] }}</span>
       </div>
+      <div class="node-stats">
+        <div class="stat-grid">
+          <div class="stat-box">
+            <span class="stat-label">连接数</span>
+            <span class="stat-value" data-role="connections">-</span>
+          </div>
+          <div class="stat-box">
+            <span class="stat-label">实时 ↑</span>
+            <span class="stat-value" data-role="speed-up">-</span>
+          </div>
+          <div class="stat-box">
+            <span class="stat-label">实时 ↓</span>
+            <span class="stat-value" data-role="speed-down">-</span>
+          </div>
+          <div class="stat-box">
+            <span class="stat-label">累计 ↑</span>
+            <span class="stat-value" data-role="total-up">-</span>
+          </div>
+          <div class="stat-box">
+            <span class="stat-label">累计 ↓</span>
+            <span class="stat-value" data-role="total-down">-</span>
+          </div>
+        </div>
+      </div>
       <div class="field">
         <label>VLESS + Reality</label>
         <div class="copy-row">