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>

config/runtime.exs

@@ -112,4 +112,13 @@ if config_env() == :prod do
   #     config :swoosh, :api_client, Swoosh.ApiClient.Req
   #
   # See https://hexdocs.pm/swoosh/Swoosh.html#module-installation for details.
+
+  # Stripe payment processing
+  config :stripity_stripe,
+    api_key:
+      System.get_env("STRIPE_SECRET_KEY") ||
+        raise("Missing STRIPE_SECRET_KEY environment variable"),
+    signing_secret:
+      System.get_env("STRIPE_WEBHOOK_SECRET") ||
+        raise("Missing STRIPE_WEBHOOK_SECRET environment variable")
 end