feat(usage): 使用记录增加计费模式字段 — 记录/展示/筛选 token/按次/图片

- DB: usage_logs 表新增 billing_mode VARCHAR(20) 列
- 后端: RecordUsage 写入时根据 image_count 判定计费模式
- 前端: 使用记录表格新增计费模式 badge 列 + 筛选下拉

frontend/src/i18n/locales/en.ts

@@ -3356,6 +3356,11 @@ export default {
       allBillingTypes: 'All Billing Types',
       billingTypeBalance: 'Balance',
       billingTypeSubscription: 'Subscription',
+      billingMode: 'Billing Mode',
+      billingModeToken: 'Token',
+      billingModePerRequest: 'Per Request',
+      billingModeImage: 'Image',
+      allBillingModes: 'All Billing Modes',
       ipAddress: 'IP',
       clickToViewBalance: 'Click to view balance history',
       failedToLoadUser: 'Failed to load user info',