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/i18n/locales/en.ts

@@ -429,6 +429,9 @@ export default {
     totalCost: 'Total Cost',
     standardCost: 'Standard',
     actualCost: 'Actual',
+    userBilled: 'User billed',
+    accountBilled: 'Account billed',
+    accountMultiplier: 'Account rate',
     avgDuration: 'Avg Duration',
     inSelectedRange: 'in selected range',
     perRequest: 'per request',
@@ -1058,6 +1061,7 @@ export default {
         concurrencyStatus: 'Concurrency',
         notes: 'Notes',
         priority: 'Priority',
+        billingRateMultiplier: 'Billing Rate',
         weight: 'Weight',
         status: 'Status',
         schedulable: 'Schedulable',
@@ -1225,6 +1229,8 @@ export default {
       concurrency: 'Concurrency',
       priority: 'Priority',
       priorityHint: 'Lower value accounts are used first',
+      billingRateMultiplier: 'Billing Rate Multiplier',
+      billingRateMultiplierHint: '>=0, 0 means free. Affects account billing only',
       expiresAt: 'Expires At',
       expiresAtHint: 'Leave empty for no expiration',
       higherPriorityFirst: 'Lower value means higher priority',