fix(usage): 用户使用记录不下发账号计费倍率

- 将 usage log DTO 拆分为用户/管理员两类
- 用户接口不返回 account_rate_multiplier/ip_address/account
- 管理员接口保留管理员字段
- 补充契约测试防止回归

frontend/src/api/admin/usage.ts

@@ -4,7 +4,7 @@
  */
 
 import { apiClient } from '../client'
-import type { UsageLog, UsageQueryParams, PaginatedResponse } from '@/types'
+import type { AdminUsageLog, UsageQueryParams, PaginatedResponse } from '@/types'
 
 // ==================== Types ====================
 
@@ -85,8 +85,8 @@ export interface AdminUsageQueryParams extends UsageQueryParams {
 export async function list(
   params: AdminUsageQueryParams,
   options?: { signal?: AbortSignal }
-): Promise<PaginatedResponse<UsageLog>> {
-  const { data } = await apiClient.get<PaginatedResponse<UsageLog>>('/admin/usage', {
+): Promise<PaginatedResponse<AdminUsageLog>> {
+  const { data } = await apiClient.get<PaginatedResponse<AdminUsageLog>>('/admin/usage', {
     params,
     signal: options?.signal
   })