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',

frontend/src/i18n/locales/zh.ts

@@ -3515,6 +3515,11 @@ export default {
       allBillingTypes: '全部计费类型',
       billingTypeBalance: '钱包余额',
       billingTypeSubscription: '订阅套餐',
+      billingMode: '计费模式',
+      billingModeToken: '按量',
+      billingModePerRequest: '按次',
+      billingModeImage: '按次(图片)',
+      allBillingModes: '全部计费模式',
       ipAddress: 'IP',
       clickToViewBalance: '点击查看充值记录',
       failedToLoadUser: '加载用户信息失败',