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/controllers/orders_controller.rb

@@ -293,18 +293,7 @@ class OrdersController < ApplicationController
       }
     end
 
-    # Add service fee as a separate line item
-    line_items << {
-      price_data: {
-        currency: "eur",
-        product_data: {
-          name: "Frais de service",
-          description: "Frais de traitement de la commande"
-        },
-        unit_amount: 100 # 1€ in cents
-      },
-      quantity: 1
-    }
+    # No service fee added to customer; deducted from promoter payout
 
     Stripe::Checkout::Session.create(
       payment_method_types: [ "card" ],