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

backend/internal/service/admin_service.go

@@ -420,6 +420,8 @@ type adminServiceImpl struct {
 	proxyLatencyCache    ProxyLatencyCache
 	authCacheInvalidator APIKeyAuthCacheInvalidator
 	entClient            *dbent.Client // 用于开启数据库事务
+	settingService       *SettingService
+	defaultSubAssigner   DefaultSubscriptionAssigner
 }
 
 type userGroupRateBatchReader interface {
@@ -445,6 +447,8 @@ func NewAdminService(
 	proxyLatencyCache ProxyLatencyCache,
 	authCacheInvalidator APIKeyAuthCacheInvalidator,
 	entClient *dbent.Client,
+	settingService *SettingService,
+	defaultSubAssigner DefaultSubscriptionAssigner,
 ) AdminService {
 	return &adminServiceImpl{
 		userRepo:             userRepo,
@@ -460,6 +464,8 @@ func NewAdminService(
 		proxyLatencyCache:    proxyLatencyCache,
 		authCacheInvalidator: authCacheInvalidator,
 		entClient:            entClient,
+		settingService:       settingService,
+		defaultSubAssigner:   defaultSubAssigner,
 	}
 }
 
@@ -544,9 +550,27 @@ func (s *adminServiceImpl) CreateUser(ctx context.Context, input *CreateUserInpu
 	if err := s.userRepo.Create(ctx, user); err != nil {
 		return nil, err
 	}
+	s.assignDefaultSubscriptions(ctx, user.ID)
 	return user, nil
 }
 
+func (s *adminServiceImpl) assignDefaultSubscriptions(ctx context.Context, userID int64) {
+	if s.settingService == nil || s.defaultSubAssigner == nil || userID <= 0 {
+		return
+	}
+	items := s.settingService.GetDefaultSubscriptions(ctx)
+	for _, item := range items {
+		if _, _, err := s.defaultSubAssigner.AssignOrExtendSubscription(ctx, &AssignSubscriptionInput{
+			UserID:       userID,
+			GroupID:      item.GroupID,
+			ValidityDays: item.ValidityDays,
+			Notes:        "auto assigned by default user subscriptions setting",
+		}); err != nil {
+			logger.LegacyPrintf("service.admin", "failed to assign default subscription: user_id=%d group_id=%d err=%v", userID, item.GroupID, err)
+		}
+	}
+}
+
 func (s *adminServiceImpl) UpdateUser(ctx context.Context, id int64, input *UpdateUserInput) (*User, error) {
 	user, err := s.userRepo.GetByID(ctx, id)
 	if err != nil {