feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

frontend/src/components/account/AccountStatsModal.vue

@@ -73,11 +73,12 @@
             </p>
             <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
               {{ t('admin.accounts.stats.accumulatedCost') }}
-              <span class="text-gray-400 dark:text-gray-500"
-                >({{ t('admin.accounts.stats.standardCost') }}: ${{
+              <span class="text-gray-400 dark:text-gray-500">
+                ({{ t('usage.userBilled') }}: ${{ formatCost(stats.summary.total_user_cost) }} ·
+                {{ t('admin.accounts.stats.standardCost') }}: ${{
                   formatCost(stats.summary.total_standard_cost)
-                }})</span
-              >
+                }})
+              </span>
             </p>
           </div>
 
@@ -121,12 +122,15 @@
             <p class="text-2xl font-bold text-gray-900 dark:text-white">
               ${{ formatCost(stats.summary.avg_daily_cost) }}
             </p>
-            <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+             <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
               {{
                 t('admin.accounts.stats.basedOnActualDays', {
                   days: stats.summary.actual_days_used
                 })
               }}
+              <span class="text-gray-400 dark:text-gray-500">
+                ({{ t('usage.userBilled') }}: ${{ formatCost(stats.summary.avg_daily_user_cost) }})
+              </span>
             </p>
           </div>
 
@@ -189,13 +193,17 @@
             </div>
             <div class="space-y-2">
               <div class="flex items-center justify-between">
-                <span class="text-xs text-gray-500 dark:text-gray-400">{{
-                  t('admin.accounts.stats.cost')
-                }}</span>
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.accountBilled') }}</span>
                 <span class="text-sm font-semibold text-gray-900 dark:text-white"
                   >${{ formatCost(stats.summary.today?.cost || 0) }}</span
                 >
               </div>
+              <div class="flex items-center justify-between">
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.userBilled') }}</span>
+                <span class="text-sm font-semibold text-gray-900 dark:text-white"
+                  >${{ formatCost(stats.summary.today?.user_cost || 0) }}</span
+                >
+              </div>
               <div class="flex items-center justify-between">
                 <span class="text-xs text-gray-500 dark:text-gray-400">{{
                   t('admin.accounts.stats.requests')
@@ -240,13 +248,17 @@
                 }}</span>
               </div>
               <div class="flex items-center justify-between">
-                <span class="text-xs text-gray-500 dark:text-gray-400">{{
-                  t('admin.accounts.stats.cost')
-                }}</span>
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.accountBilled') }}</span>
                 <span class="text-sm font-semibold text-orange-600 dark:text-orange-400"
                   >${{ formatCost(stats.summary.highest_cost_day?.cost || 0) }}</span
                 >
               </div>
+              <div class="flex items-center justify-between">
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.userBilled') }}</span>
+                <span class="text-sm font-semibold text-gray-900 dark:text-white"
+                  >${{ formatCost(stats.summary.highest_cost_day?.user_cost || 0) }}</span
+                >
+              </div>
               <div class="flex items-center justify-between">
                 <span class="text-xs text-gray-500 dark:text-gray-400">{{
                   t('admin.accounts.stats.requests')
@@ -291,13 +303,17 @@
                 }}</span>
               </div>
               <div class="flex items-center justify-between">
-                <span class="text-xs text-gray-500 dark:text-gray-400">{{
-                  t('admin.accounts.stats.cost')
-                }}</span>
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.accountBilled') }}</span>
                 <span class="text-sm font-semibold text-gray-900 dark:text-white"
                   >${{ formatCost(stats.summary.highest_request_day?.cost || 0) }}</span
                 >
               </div>
+              <div class="flex items-center justify-between">
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.userBilled') }}</span>
+                <span class="text-sm font-semibold text-gray-900 dark:text-white"
+                  >${{ formatCost(stats.summary.highest_request_day?.user_cost || 0) }}</span
+                >
+              </div>
             </div>
           </div>
         </div>
@@ -397,13 +413,17 @@
                 }}</span>
               </div>
               <div class="flex items-center justify-between">
-                <span class="text-xs text-gray-500 dark:text-gray-400">{{
-                  t('admin.accounts.stats.todayCost')
-                }}</span>
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.accountBilled') }}</span>
                 <span class="text-sm font-semibold text-gray-900 dark:text-white"
                   >${{ formatCost(stats.summary.today?.cost || 0) }}</span
                 >
               </div>
+              <div class="flex items-center justify-between">
+                <span class="text-xs text-gray-500 dark:text-gray-400">{{ t('usage.userBilled') }}</span>
+                <span class="text-sm font-semibold text-gray-900 dark:text-white"
+                  >${{ formatCost(stats.summary.today?.user_cost || 0) }}</span
+                >
+              </div>
             </div>
           </div>
         </div>
@@ -517,14 +537,24 @@ const trendChartData = computed(() => {
     labels: stats.value.history.map((h) => h.label),
     datasets: [
       {
-        label: t('admin.accounts.stats.cost') + ' (USD)',
-        data: stats.value.history.map((h) => h.cost),
+        label: t('usage.accountBilled') + ' (USD)',
+        data: stats.value.history.map((h) => h.actual_cost),
         borderColor: '#3b82f6',
         backgroundColor: 'rgba(59, 130, 246, 0.1)',
         fill: true,
         tension: 0.3,
         yAxisID: 'y'
       },
+      {
+        label: t('usage.userBilled') + ' (USD)',
+        data: stats.value.history.map((h) => h.user_cost),
+        borderColor: '#10b981',
+        backgroundColor: 'rgba(16, 185, 129, 0.08)',
+        fill: false,
+        tension: 0.3,
+        borderDash: [5, 5],
+        yAxisID: 'y'
+      },
       {
         label: t('admin.accounts.stats.requests'),
         data: stats.value.history.map((h) => h.requests),
@@ -602,7 +632,7 @@ const lineChartOptions = computed(() => ({
       },
       title: {
         display: true,
-        text: t('admin.accounts.stats.cost') + ' (USD)',
+        text: t('usage.accountBilled') + ' (USD)',
         color: '#3b82f6',
         font: {
           size: 11