feat: add transactional emails for order confirmation and shipping

Plain text emails via Swoosh OrderNotifier module. Order confirmation
triggered from Stripe webhook after payment, shipping notification
from Printify shipment webhook with polling fallback.

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

lib/simpleshop_theme_web/controllers/stripe_webhook_controller.ex

@@ -2,7 +2,7 @@ defmodule SimpleshopThemeWeb.StripeWebhookController do
   use SimpleshopThemeWeb, :controller
 
   alias SimpleshopTheme.Orders
-  alias SimpleshopTheme.Orders.OrderSubmissionWorker
+  alias SimpleshopTheme.Orders.{OrderNotifier, OrderSubmissionWorker}
 
   require Logger
 
@@ -56,6 +56,9 @@ defmodule SimpleshopThemeWeb.StripeWebhookController do
             order
           end
 
+        # Reload items for the email (update_order doesn't preload)
+        order = Orders.get_order(order.id)
+
         # Broadcast to success page via PubSub
         Phoenix.PubSub.broadcast(
           SimpleshopTheme.PubSub,
@@ -63,6 +66,8 @@ defmodule SimpleshopThemeWeb.StripeWebhookController do
           {:order_paid, order}
         )
 
+        OrderNotifier.deliver_order_confirmation(order)
+
         # Submit to fulfilment provider
         if order.shipping_address && order.shipping_address != %{} do
           OrderSubmissionWorker.enqueue(order.id)