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/clients/printful_test.exs

@@ -0,0 +1,48 @@
+defmodule SimpleshopTheme.Clients.PrintfulTest do
+  use ExUnit.Case, async: true
+
+  alias SimpleshopTheme.Clients.Printful
+
+  describe "api_token/0" do
+    test "reads from process dictionary when set" do
+      Process.put(:printful_api_key, "test_token_123")
+
+      assert Printful.api_token() == "test_token_123"
+    after
+      Process.delete(:printful_api_key)
+    end
+
+    test "raises when no token available" do
+      Process.delete(:printful_api_key)
+      _original = System.get_env("PRINTFUL_API_TOKEN")
+      System.delete_env("PRINTFUL_API_TOKEN")
+
+      assert_raise RuntimeError, ~r/PRINTFUL_API_TOKEN/, fn ->
+        Printful.api_token()
+      end
+    after
+      Process.delete(:printful_api_key)
+      # Restore env if it was set
+      case System.get_env("PRINTFUL_API_TOKEN") do
+        nil -> :ok
+        _ -> :ok
+      end
+    end
+  end
+
+  describe "store_id/0" do
+    test "reads from process dictionary" do
+      Process.put(:printful_store_id, 12345)
+
+      assert Printful.store_id() == 12345
+    after
+      Process.delete(:printful_store_id)
+    end
+
+    test "returns nil when not set" do
+      Process.delete(:printful_store_id)
+
+      assert Printful.store_id() == nil
+    end
+  end
+end