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>

ROADMAP.md

@@ -4,35 +4,14 @@
 
 ---
 
-## Core MVP: Cart & Checkout
+## Core MVP: Cart & Checkout ✅
 
-### Session-Based Cart
-Store cart in Phoenix session (no separate table needed for MVP).
+Session-based cart, Stripe-hosted Checkout, order persistence, and webhook handling are all complete. See [PROGRESS.md](PROGRESS.md) for details.
 
-```elixir
-defmodule SimpleshopTheme.Cart do
-  def get(session), do: Map.get(session, "cart", %{})
-  def add_item(session, variant_id, quantity \\ 1)
-  def remove_item(session, variant_id)
-  def update_quantity(session, variant_id, quantity)
-  def clear(session)
-  def to_line_items(cart)
-  def total(cart)
-  def item_count(cart)
-end
-```
-
-### Stripe Checkout
-Stripe Checkout (hosted payment page) integration.
-
-**Dependencies:** `{:stripity_stripe, "~> 3.0"}`
-
-**Routes:**
-```elixir
-post "/webhooks/stripe", StripeWebhookController, :handle
-live "/checkout/success", ShopLive.CheckoutSuccess
-live "/checkout/cancel", ShopLive.CheckoutCancel
-```
+### Orders & Fulfillment (next up)
+- Submit paid orders to Printify for fulfillment
+- Track fulfillment status updates via webhook
+- Display order status to customers
 
 ### Cost Verification at Checkout
 Verify Printify costs haven't changed before completing checkout to prevent selling at a loss.