add Printful provider integration with HTTP client and order routing

Printful HTTP client (v2 + v1 for sync products), Provider behaviour
implementation with all callbacks (test_connection, fetch_products,
submit_order, get_order_status, fetch_shipping_rates), and multi-provider
order routing that looks up the provider connection from the order's
product instead of hardcoding "printify".

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

test/simpleshop_theme/orders/order_submission_worker_test.exs

@@ -84,7 +84,9 @@ defmodule SimpleshopTheme.Orders.OrderSubmissionWorkerTest do
     end
 
     test "sets failed status when variant not found in local DB" do
-      # Create order with a variant_id that doesn't exist in product variants
+      # Create order with a variant_id that doesn't exist in product variants.
+      # Provider connection lookup now goes through the variant's product,
+      # so a missing variant causes :variant_not_found at routing time.
       order =
         order_fixture(%{
           variant_id: Ecto.UUID.generate(),
@@ -98,10 +100,7 @@ defmodule SimpleshopTheme.Orders.OrderSubmissionWorkerTest do
           }
         })
 
-      # Need a printify connection for the lookup to proceed
-      SimpleshopTheme.ProductsFixtures.provider_connection_fixture(%{provider_type: "printify"})
-
-      assert {:error, {:variant_not_found, _, _}} =
+      assert {:error, :variant_not_found} =
                OrderSubmissionWorker.perform(%Oban.Job{args: %{"order_id" => order.id}})
 
       updated = Orders.get_order(order.id)