Fix service fee missing from Stripe invoices

The StripeInvoiceService was only creating line items for tickets but missing
the 1€ service fee, causing a discrepancy where customers paid 26€ via Stripe
checkout but the generated invoice only showed 25€.

- Add service fee line item to Stripe invoices in StripeInvoiceService
- Update all related tests to expect two line items (tickets + service fee)
- Fix order controller test to account for service fee in total calculation

Now Stripe invoices properly match the amount paid: tickets + 1€ service fee.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/services/stripe_invoice_service.rb

@@ -147,6 +147,7 @@ class StripeInvoiceService
   end
 
   def add_line_items_to_invoice(customer, invoice)
+    # Add ticket line items
     @order.tickets.group_by(&:ticket_type).each do |ticket_type, tickets|
       quantity = tickets.count
 
@@ -164,6 +165,20 @@ class StripeInvoiceService
         }
       })
     end
+
+    # Add service fee line item
+    service_fee_cents = 100 # 1€ service fee
+    Stripe::InvoiceItem.create({
+      customer: customer.id,
+      invoice: invoice.id,
+      amount: service_fee_cents,
+      currency: "eur",
+      description: "Frais de service - Frais de traitement de la commande",
+      metadata: {
+        item_type: "service_fee",
+        amount_cents: service_fee_cents
+      }
+    })
   end
 
   def build_line_item_description(ticket_type, tickets)

test/controllers/orders_controller_test.rb

@@ -122,7 +122,7 @@ class OrdersControllerTest < ActionDispatch::IntegrationTest
     assert_equal "draft", new_order.status
     assert_equal @user, new_order.user
     assert_equal @event, new_order.event
-    assert_equal @ticket_type.price_cents, new_order.total_amount_cents
+    assert_equal @ticket_type.price_cents + 100, new_order.total_amount_cents # includes 1€ service fee
 
     assert_redirected_to checkout_order_path(new_order)
     assert_equal new_order.id, session[:draft_order_id]

test/services/stripe_invoice_service_test.rb

@@ -151,7 +151,7 @@ class StripeInvoiceServiceTest < ActiveSupport::TestCase
     mock_invoice.stubs(:finalize_invoice).returns(mock_invoice)
     mock_invoice.expects(:pay)
     Stripe::Invoice.expects(:create).returns(mock_invoice)
-    Stripe::InvoiceItem.expects(:create).once
+    Stripe::InvoiceItem.expects(:create).twice # Once for tickets, once for service fee
 
     result = @service.create_post_payment_invoice
     assert_not_nil result
@@ -173,7 +173,7 @@ class StripeInvoiceServiceTest < ActiveSupport::TestCase
     mock_invoice.stubs(:finalize_invoice).returns(mock_invoice)
     mock_invoice.expects(:pay)
     Stripe::Invoice.expects(:create).returns(mock_invoice)
-    Stripe::InvoiceItem.expects(:create).once
+    Stripe::InvoiceItem.expects(:create).twice # Once for tickets, once for service fee
 
     result = @service.create_post_payment_invoice
     assert_not_nil result
@@ -196,7 +196,7 @@ class StripeInvoiceServiceTest < ActiveSupport::TestCase
     mock_customer.stubs(:id).returns("cus_test123")
     Stripe::Customer.expects(:create).returns(mock_customer)
 
-    expected_line_item = {
+    expected_ticket_line_item = {
       customer: "cus_test123",
       invoice: "in_test123",
       amount: @ticket_type.price_cents * 2, # 2 tickets
@@ -210,12 +210,25 @@ class StripeInvoiceServiceTest < ActiveSupport::TestCase
       }
     }
 
+    expected_service_fee_line_item = {
+      customer: "cus_test123",
+      invoice: "in_test123",
+      amount: 100,
+      currency: "eur",
+      description: "Frais de service - Frais de traitement de la commande",
+      metadata: {
+        item_type: "service_fee",
+        amount_cents: 100
+      }
+    }
+
     mock_invoice = mock("invoice")
     mock_invoice.stubs(:id).returns("in_test123")
     mock_invoice.stubs(:finalize_invoice).returns(mock_invoice)
     mock_invoice.expects(:pay)
     Stripe::Invoice.expects(:create).returns(mock_invoice)
-    Stripe::InvoiceItem.expects(:create).with(expected_line_item)
+    Stripe::InvoiceItem.expects(:create).with(expected_ticket_line_item)
+    Stripe::InvoiceItem.expects(:create).with(expected_service_fee_line_item)
 
     result = @service.create_post_payment_invoice
     assert_not_nil result
@@ -248,7 +261,7 @@ class StripeInvoiceServiceTest < ActiveSupport::TestCase
     mock_invoice.expects(:pay)
 
     Stripe::Invoice.expects(:create).with(expected_invoice_data).returns(mock_invoice)
-    Stripe::InvoiceItem.expects(:create).once
+    Stripe::InvoiceItem.expects(:create).twice # Once for tickets, once for service fee
 
     result = @service.create_post_payment_invoice
     assert_not_nil result
@@ -287,7 +300,7 @@ class StripeInvoiceServiceTest < ActiveSupport::TestCase
     })
 
     Stripe::Invoice.expects(:create).returns(mock_invoice)
-    Stripe::InvoiceItem.expects(:create).once
+    Stripe::InvoiceItem.expects(:create).twice # Once for tickets, once for service fee
     mock_invoice.expects(:finalize_invoice).returns(mock_finalized_invoice)
 
     result = @service.create_post_payment_invoice