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>

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