feat(ui): 用户列表页显示当前并发数

优化 /admin/users 页面的并发数列，显示「当前/最大」格式，
参考 AccountCapacityCell 的设计风格。

- 后端 UserHandler 注入 ConcurrencyService，批量查询用户当前并发数
- 新增 UserConcurrencyCell 组件，支持颜色状态（空闲灰/使用中黄/满载红）
- 前端 AdminUser 类型添加 current_concurrency 字段

frontend/src/components/user/UserConcurrencyCell.vue

@@ -0,0 +1,43 @@
+<template>
+  <div class="flex items-center">
+    <span
+      :class="[
+        'inline-flex items-center gap-1 rounded-md px-2 py-0.5 text-xs font-medium',
+        statusClass
+      ]"
+    >
+      <!-- Four-square grid icon -->
+      <svg class="h-3 w-3" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2">
+        <path stroke-linecap="round" stroke-linejoin="round" d="M3.75 6A2.25 2.25 0 016 3.75h2.25A2.25 2.25 0 0110.5 6v2.25a2.25 2.25 0 01-2.25 2.25H6a2.25 2.25 0 01-2.25-2.25V6zM3.75 15.75A2.25 2.25 0 016 13.5h2.25a2.25 2.25 0 012.25 2.25V18a2.25 2.25 0 01-2.25 2.25H6A2.25 2.25 0 013.75 18v-2.25zM13.5 6a2.25 2.25 0 012.25-2.25H18A2.25 2.25 0 0120.25 6v2.25A2.25 2.25 0 0118 10.5h-2.25a2.25 2.25 0 01-2.25-2.25V6zM13.5 15.75a2.25 2.25 0 012.25-2.25H18a2.25 2.25 0 012.25 2.25V18A2.25 2.25 0 0118 20.25h-2.25A2.25 2.25 0 0113.5 18v-2.25z" />
+      </svg>
+      <span class="font-mono">{{ current }}</span>
+      <span class="text-gray-400 dark:text-gray-500">/</span>
+      <span class="font-mono">{{ max }}</span>
+    </span>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue'
+
+const props = defineProps<{
+  current: number
+  max: number
+}>()
+
+// Status color based on usage
+const statusClass = computed(() => {
+  const { current, max } = props
+
+  // Full: red
+  if (current >= max && max > 0) {
+    return 'bg-red-100 text-red-700 dark:bg-red-900/30 dark:text-red-400'
+  }
+  // In use: yellow
+  if (current > 0) {
+    return 'bg-yellow-100 text-yellow-700 dark:bg-yellow-900/30 dark:text-yellow-400'
+  }
+  // Idle: gray
+  return 'bg-gray-100 text-gray-600 dark:bg-gray-800 dark:text-gray-400'
+})
+</script>