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/views/admin/AccountsView.vue

@@ -61,6 +61,11 @@
           <template #cell-usage="{ row }">
             <AccountUsageCell :account="row" />
           </template>
+          <template #cell-rate_multiplier="{ row }">
+            <span class="text-sm font-mono text-gray-700 dark:text-gray-300">
+              {{ (row.rate_multiplier ?? 1).toFixed(2) }}x
+            </span>
+          </template>
           <template #cell-priority="{ value }">
             <span class="text-sm text-gray-700 dark:text-gray-300">{{ value }}</span>
           </template>
@@ -190,10 +195,11 @@ const cols = computed(() => {
   if (!authStore.isSimpleMode) {
     c.push({ key: 'groups', label: t('admin.accounts.columns.groups'), sortable: false })
   }
-  c.push(
-    { key: 'usage', label: t('admin.accounts.columns.usageWindows'), sortable: false },
-    { key: 'priority', label: t('admin.accounts.columns.priority'), sortable: true },
-    { key: 'last_used_at', label: t('admin.accounts.columns.lastUsed'), sortable: true },
+    c.push(
+      { key: 'usage', label: t('admin.accounts.columns.usageWindows'), sortable: false },
+      { key: 'priority', label: t('admin.accounts.columns.priority'), sortable: true },
+      { key: 'rate_multiplier', label: t('admin.accounts.columns.billingRateMultiplier'), sortable: true },
+      { key: 'last_used_at', label: t('admin.accounts.columns.lastUsed'), sortable: true },
     { key: 'expires_at', label: t('admin.accounts.columns.expiresAt'), sortable: true },
     { key: 'notes', label: t('admin.accounts.columns.notes'), sortable: false },
     { key: 'actions', label: t('admin.accounts.columns.actions'), sortable: false }

frontend/src/views/admin/UsageView.vue

@@ -94,7 +94,7 @@ const exportToExcel = async () => {
         t('admin.usage.cacheReadTokens'), t('admin.usage.cacheCreationTokens'),
         t('admin.usage.inputCost'), t('admin.usage.outputCost'),
         t('admin.usage.cacheReadCost'), t('admin.usage.cacheCreationCost'),
-        t('usage.rate'), t('usage.original'), t('usage.billed'),
+        t('usage.rate'), t('usage.accountMultiplier'), t('usage.original'), t('usage.userBilled'), t('usage.accountBilled'),
         t('usage.firstToken'), t('usage.duration'),
         t('admin.usage.requestId'), t('usage.userAgent'), t('admin.usage.ipAddress')
       ]
@@ -115,8 +115,10 @@ const exportToExcel = async () => {
         log.cache_read_cost?.toFixed(6) || '0.000000',
         log.cache_creation_cost?.toFixed(6) || '0.000000',
         log.rate_multiplier?.toFixed(2) || '1.00',
+        (log.account_rate_multiplier ?? 1).toFixed(2),
         log.total_cost?.toFixed(6) || '0.000000',
         log.actual_cost?.toFixed(6) || '0.000000',
+        (log.total_cost * (log.account_rate_multiplier ?? 1)).toFixed(6),
         log.first_token_ms ?? '',
         log.duration_ms,
         log.request_id || '',