feat(settings): add default subscriptions for new users

- add default subscriptions to admin settings

- auto-assign subscriptions on register and admin user creation

- add validation/tests and align settings UI with subscription selector patterns

frontend/src/api/admin/settings.ts

@@ -5,6 +5,11 @@
 
 import { apiClient } from '../client'
 
+export interface DefaultSubscriptionSetting {
+  group_id: number
+  validity_days: number
+}
+
 /**
  * System settings interface
  */
@@ -20,6 +25,7 @@ export interface SystemSettings {
   // Default settings
   default_balance: number
   default_concurrency: number
+  default_subscriptions: DefaultSubscriptionSetting[]
   // OEM settings
   site_name: string
   site_logo: string
@@ -81,6 +87,7 @@ export interface UpdateSettingsRequest {
   totp_enabled?: boolean // TOTP 双因素认证
   default_balance?: number
   default_concurrency?: number
+  default_subscriptions?: DefaultSubscriptionSetting[]
   site_name?: string
   site_logo?: string
   site_subtitle?: string