Add public refund creation endpoint and PDF generation

Introduces RefundPublicView for public refund creation via email and invoice/order ID, returning refund info and a base64-encoded PDF slip. Adds RefundCreatePublicSerializer for validation and creation, implements PDF generation in Refund model, and provides a customer-facing HTML template for the return slip. Updates URLs to expose the new endpoint.

backend/commerce/urls.py

@@ -7,6 +7,7 @@ from .views import (
     ProductImageViewSet,
     DiscountCodeViewSet,
     RefundViewSet,
+    RefundPublicView,
 )
 
 router = DefaultRouter()
@@ -19,6 +20,7 @@ router.register(r'refunds', RefundViewSet, basename='refund')
 
 urlpatterns = [
     path('', include(router.urls)),
+    path('refunds/public/', RefundPublicView.as_view(), name='RefundPublicView'),
 ]
 
 # NOTE: Other endpoints (categories/products/discounts) can be added later