feat(admin/usage): 优化管理员用量页面功能和体验

后端改进：
- 新增 GetStatsWithFilters 方法支持完整筛选条件
- Stats 端点支持 account_id, group_id, model, stream, billing_type 参数
- 统一使用 filters 结构体，移除冗余的分支逻辑

前端改进：
- 统计卡片添加"所选范围内"文字提示
- 优化总消费显示格式，清晰展示实际费用和标准计费
- Token 和费用列添加问号图标 tooltip 显示详细信息
- API Key 搜索框体验优化：点击即显示下拉选项
- 选择用户后自动加载该用户的所有 API Key

frontend/src/components/admin/usage/UsageStatsCards.vue

@@ -4,17 +4,34 @@
       <div class="rounded-lg bg-blue-100 p-2 dark:bg-blue-900/30 text-blue-600">
         <Icon name="document" size="md" />
       </div>
-      <div><p class="text-xs font-medium text-gray-500">{{ t('usage.totalRequests') }}</p><p class="text-xl font-bold">{{ stats?.total_requests?.toLocaleString() || '0' }}</p></div>
+      <div>
+        <p class="text-xs font-medium text-gray-500">{{ t('usage.totalRequests') }}</p>
+        <p class="text-xl font-bold">{{ stats?.total_requests?.toLocaleString() || '0' }}</p>
+        <p class="text-xs text-gray-400">{{ t('usage.inSelectedRange') }}</p>
+      </div>
     </div>
     <div class="card p-4 flex items-center gap-3">
       <div class="rounded-lg bg-amber-100 p-2 dark:bg-amber-900/30 text-amber-600"><svg class="h-5 w-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="m21 7.5-9-5.25L3 7.5m18 0-9 5.25m9-5.25v9l-9 5.25M3 7.5l9 5.25M3 7.5v9l9 5.25m0-9v9" /></svg></div>
-      <div><p class="text-xs font-medium text-gray-500">{{ t('usage.totalTokens') }}</p><p class="text-xl font-bold">{{ formatTokens(stats?.total_tokens || 0) }}</p></div>
+      <div>
+        <p class="text-xs font-medium text-gray-500">{{ t('usage.totalTokens') }}</p>
+        <p class="text-xl font-bold">{{ formatTokens(stats?.total_tokens || 0) }}</p>
+        <p class="text-xs text-gray-500">
+          {{ t('usage.in') }}: {{ formatTokens(stats?.total_input_tokens || 0) }} /
+          {{ t('usage.out') }}: {{ formatTokens(stats?.total_output_tokens || 0) }}
+        </p>
+      </div>
     </div>
     <div class="card p-4 flex items-center gap-3">
       <div class="rounded-lg bg-green-100 p-2 dark:bg-green-900/30 text-green-600">
         <Icon name="dollar" size="md" />
       </div>
-      <div><p class="text-xs font-medium text-gray-500">{{ t('usage.totalCost') }}</p><p class="text-xl font-bold text-green-600">${{ (stats?.total_actual_cost || 0).toFixed(4) }}</p></div>
+      <div class="min-w-0 flex-1">
+        <p class="text-xs font-medium text-gray-500">{{ t('usage.totalCost') }}</p>
+        <p class="text-xl font-bold text-green-600">${{ (stats?.total_actual_cost || 0).toFixed(4) }}</p>
+        <p class="text-xs text-gray-400">
+          {{ t('usage.standardCost') }}: <span class="line-through">${{ (stats?.total_cost || 0).toFixed(4) }}</span>
+        </p>
+      </div>
     </div>
     <div class="card p-4 flex items-center gap-3">
       <div class="rounded-lg bg-purple-100 p-2 dark:bg-purple-900/30 text-purple-600">