[add] added maximum client and demo client protection

- default demo clients can no longer be modified/deleted when in demo mode
 - increased default demo reset to 60 minutes

spec/requests/clients_request_spec.rb

@@ -160,6 +160,20 @@ RSpec.describe 'Clients', type: :request do
         expect(response).to have_http_status(200)
       end
     end
+
+    context 'when sending a valid request to add another client above the MAX_CLIENTS limit' do
+      before do
+        clients_to_create =  RynDNS::Application::MAX_CLIENTS - Client.count
+        create_list(:client, clients_to_create)
+        post '/admin/client', params: { name: 'extra_user' }, headers: api_key_header(admin_client)
+      end
+
+      it 'does not add another client and returns status code 429' do
+        expect(Client.count).to eq RynDNS::Application::MAX_CLIENTS
+        expect(response.body).to be_empty
+        expect(response).to have_http_status(429)
+      end
+    end
   end
 
   describe 'PUT /admin/client' do