feat: add Stripe checkout, order persistence, and webhook handling

Stripe-hosted Checkout integration with full order lifecycle:
- stripity_stripe ~> 3.2 with sandbox/prod config via env vars
- Order and OrderItem schemas with price snapshots at purchase time
- CheckoutController creates pending order then redirects to Stripe
- StripeWebhookController verifies signatures and confirms payment
- Success page with real-time PubSub updates from webhook
- Shop flash messages for checkout error feedback
- Cart cleared after successful payment

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

lib/simpleshop_theme_web/live/shop_live/checkout_success.ex

@@ -0,0 +1,82 @@
+defmodule SimpleshopThemeWeb.ShopLive.CheckoutSuccess do
+  use SimpleshopThemeWeb, :live_view
+
+  alias SimpleshopTheme.{Orders, Settings, Media}
+  alias SimpleshopTheme.Theme.{CSSCache, CSSGenerator}
+
+  @impl true
+  def mount(%{"session_id" => session_id}, _session, socket) do
+    theme_settings = Settings.get_theme_settings()
+
+    generated_css =
+      case CSSCache.get() do
+        {:ok, css} ->
+          css
+
+        :miss ->
+          css = CSSGenerator.generate(theme_settings)
+          CSSCache.put(css)
+          css
+      end
+
+    logo_image = Media.get_logo()
+    header_image = Media.get_header()
+
+    order = Orders.get_order_by_stripe_session(session_id)
+
+    # Subscribe to order status updates (webhook may arrive after redirect)
+    if order && connected?(socket) do
+      Phoenix.PubSub.subscribe(SimpleshopTheme.PubSub, "order:#{order.id}:status")
+    end
+
+    # Clear the cart after successful checkout
+    socket =
+      if order && connected?(socket) do
+        empty_cart = []
+
+        socket
+        |> SimpleshopThemeWeb.CartHook.broadcast_and_update(empty_cart)
+      else
+        socket
+      end
+
+    socket =
+      socket
+      |> assign(:page_title, "Order confirmed")
+      |> assign(:theme_settings, theme_settings)
+      |> assign(:generated_css, generated_css)
+      |> assign(:logo_image, logo_image)
+      |> assign(:header_image, header_image)
+      |> assign(:mode, :shop)
+      |> assign(:order, order)
+
+    {:ok, socket}
+  end
+
+  def mount(_params, _session, socket) do
+    {:ok, redirect(socket, to: ~p"/")}
+  end
+
+  @impl true
+  def handle_info({:order_paid, order}, socket) do
+    {:noreply, assign(socket, :order, order)}
+  end
+
+  @impl true
+  def render(assigns) do
+    ~H"""
+    <SimpleshopThemeWeb.PageTemplates.checkout_success
+      theme_settings={@theme_settings}
+      logo_image={@logo_image}
+      header_image={@header_image}
+      mode={@mode}
+      cart_items={@cart_items}
+      cart_count={@cart_count}
+      cart_subtotal={@cart_subtotal}
+      cart_drawer_open={@cart_drawer_open}
+      cart_status={@cart_status}
+      order={@order}
+    />
+    """
+  end
+end