fix(api): use correct instance setting method in user registration

Replace non-existent GetWorkspaceGeneralSetting with GetInstanceGeneralSetting
to properly check if user registration is allowed. This fixes a compilation
error that was preventing tests from running.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>

server/router/api/v1/user_service.go

@@ -172,11 +172,11 @@ func (s *APIV1Service) CreateUser(ctx context.Context, request *v1pb.CreateUserR
 
 		// Only allow user registration if it is enabled in the settings, or if the user is a superuser
 		if currentUser == nil || !isSuperUser(currentUser) {
-			workspaceGeneralSetting, err := s.Store.GetWorkspaceGeneralSetting(ctx)
+			instanceGeneralSetting, err := s.Store.GetInstanceGeneralSetting(ctx)
 			if err != nil {
-				return nil, status.Errorf(codes.Internal, "failed to get workspace general setting, error: %v", err)
+				return nil, status.Errorf(codes.Internal, "failed to get instance general setting, error: %v", err)
 			}
-			if workspaceGeneralSetting.DisallowUserRegistration {
+			if instanceGeneralSetting.DisallowUserRegistration {
 				return nil, status.Errorf(codes.PermissionDenied, "user registration is not allowed")
 			}
 		}