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)