feat: Internal invoice generation

- TODO: make use of Stripe invoice

test/controllers/orders_controller_invoice_test.rb

@@ -0,0 +1,23 @@
+require "test_helper"
+
+class OrdersControllerInvoiceTest < ActionDispatch::IntegrationTest
+  def setup
+    @user = users(:one)
+    @event = events(:concert_event)
+    @order = orders(:paid_order)
+    sign_in @user
+  end
+
+  test "should get invoice for paid order" do
+    get invoice_order_url(@order)
+    assert_response :success
+    assert_select "h1", "Facture"
+  end
+
+  test "should redirect to order page for unpaid order" do
+    draft_order = orders(:draft_order)
+    get invoice_order_url(draft_order)
+    assert_redirected_to order_url(draft_order)
+    assert_equal "La facture n'est disponible qu'après le paiement de la commande", flash[:alert]
+  end
+end