refactor(pricing): implement hybrid fee model (€0.50 + 1.5%) deducted from promoter payout

- Remove 1€ fixed fee from orders and Stripe invoices
- Add platform_fee_cents, promoter_payout_cents methods to Order model
- Update views to show clean ticket totals without added fees
- Update tests for new fee calculation logic
- Update pricing docs with implemented model

app/services/stripe_invoice_service.rb

@@ -166,19 +166,7 @@ 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
-      }
-    })
+    # No service fee on customer invoice; platform fee deducted from promoter payout
   end
 
   def build_line_item_description(ticket_type, tickets)