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/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 || '',