fix: use correct column bound_at in admin portal

@@ -41,7 +41,8 @@ if ($pdo && $_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['action']) &&
                 $check->execute([$user['id'], $deviceId]);
 
                 if (!$check->fetch()) {
-                    $bind = $pdo->prepare("INSERT INTO user_device_bindings (user_id, device_id, is_primary, created_at) VALUES (?, ?, ?, NOW())");
+                    // FIX: use 'bound_at' instead of 'created_at'
+                    $bind = $pdo->prepare("INSERT INTO user_device_bindings (user_id, device_id, is_primary, bound_at) VALUES (?, ?, ?, NOW())");
                     $bind->execute([$user['id'], $deviceId, $isPrimary]);
                     $message = "<div style='color: green; margin-bottom: 20px; background: #e6fffa; padding: 10px; border-radius: 4px;'>✅ 成功将设备 <b>$deviceId</b> 绑定给用户 <b>{$user['nickname']}</b></div>";
                 } else {
@@ -56,15 +57,16 @@ if ($pdo && $_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['action']) &&
     }
 }
 
-// 获取设备列表 (Mock Data + Binding Count)
+// 获取设备列表
 $bindings = [];
 if ($pdo) {
     try {
+        // FIX: use 'bound_at' instead of 'created_at'
         $bindings = $pdo->query("
-            SELECT b.id, u.nickname, u.phone, b.device_id, b.is_primary, b.created_at
+            SELECT b.id, u.nickname, u.phone, b.device_id, b.is_primary, b.bound_at
             FROM user_device_bindings b
             JOIN users u ON b.user_id = u.id
-            ORDER BY b.created_at DESC
+            ORDER BY b.bound_at DESC
         ")->fetchAll(PDO::FETCH_ASSOC);
     } catch (Exception $e) {
         if (empty($message))
@@ -152,7 +154,7 @@ if ($pdo) {
                                         <span style="color: var(--primary-color);">✔</span>
                                     <?php endif; ?>
                                 </td>
-                                <td><?php echo $row['created_at']; ?></td>
+                                <td><?php echo $row['bound_at']; ?></td>
                                 <td>
                                     <a href="#" style="color: red; font-size: 12px; text-decoration: none;">解绑</a>
                                 </td>