refactor(settings): 简化流超时配置，移除冗余字段

- 移除 TimeoutSeconds 字段，超时判定由网关配置控制
- 默认禁用流超时处理功能

frontend/src/api/admin/settings.ts

@@ -206,7 +206,6 @@ export async function deleteAdminApiKey(): Promise<{ message: string }> {
  */
 export interface StreamTimeoutSettings {
   enabled: boolean
-  timeout_seconds: number
   action: 'temp_unsched' | 'error' | 'none'
   temp_unsched_minutes: number
   threshold_count: number