wire Printful shipping rates into cart calculation

Add blueprint_id and print_provider_id to Printful provider_data so the
generic shipping calculator can look up rates. Fix v2 API request format
(order_items key) and response field names. Fetch one representative
variant per product to get accurate per-item rates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/simpleshop_theme/providers/printful_test.exs

@@ -178,6 +178,8 @@ defmodule SimpleshopTheme.Providers.PrintfulTest do
 
       # Provider data
       assert normalized.provider_data.catalog_product_id == 71
+      assert normalized.provider_data.blueprint_id == 71
+      assert normalized.provider_data.print_provider_id == 0
       assert is_list(normalized.provider_data.options)
       assert length(normalized.provider_data.catalog_variant_ids) == 4
     end
@@ -357,6 +359,8 @@ defmodule SimpleshopTheme.Providers.PrintfulTest do
       provider_data: %{
         catalog_product_id: catalog_product_id,
         catalog_variant_ids: catalog_variant_ids,
+        blueprint_id: catalog_product_id,
+        print_provider_id: 0,
         thumbnail_url: sync_product["thumbnail_url"],
         options: build_option_types(sync_variants),
         raw: %{sync_product: sync_product}