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',

frontend/src/i18n/locales/zh.ts

@@ -426,6 +426,9 @@ export default {
     totalCost: '总消费',
     standardCost: '标准',
     actualCost: '实际',
+    userBilled: '用户扣费',
+    accountBilled: '账号计费',
+    accountMultiplier: '账号倍率',
     avgDuration: '平均耗时',
     inSelectedRange: '所选范围内',
     perRequest: '每次请求',
@@ -1108,6 +1111,7 @@ export default {
         concurrencyStatus: '并发',
         notes: '备注',
         priority: '优先级',
+        billingRateMultiplier: '账号倍率',
         weight: '权重',
         status: '状态',
         schedulable: '调度',
@@ -1359,6 +1363,8 @@ export default {
       concurrency: '并发数',
       priority: '优先级',
       priorityHint: '优先级越小的账号优先使用',
+      billingRateMultiplier: '账号计费倍率',
+      billingRateMultiplierHint: '>=0，0 表示该账号计费为 0；仅影响账号计费口径',
       expiresAt: '过期时间',
       expiresAtHint: '留空表示不过期',
       higherPriorityFirst: '数值越小优先级越高',