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/config.exs

@@ -83,6 +83,9 @@ config :phoenix, :json_library, Jason
 # ex_money configuration for currency handling
 config :ex_money, default_cldr_backend: SimpleshopTheme.Cldr
 
+# Stripe configuration
+config :stripity_stripe, api_version: "2024-12-18.acacia"
+
 # Oban configuration for background jobs
 config :simpleshop_theme, Oban,
   engine: Oban.Engines.Lite,
@@ -91,7 +94,7 @@ config :simpleshop_theme, Oban,
     {Oban.Plugins.Pruner, max_age: 60},
     {Oban.Plugins.Lifeline, rescue_after: :timer.minutes(5)}
   ],
-  queues: [images: 2, sync: 1]
+  queues: [images: 2, sync: 1, checkout: 1]
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.

config/dev.exs

@@ -86,3 +86,8 @@ config :phoenix_live_view,
 
 # Disable swoosh api client as it is only required for production adapters.
 config :swoosh, :api_client, false
+
+# Stripe test keys (set via environment variables)
+config :stripity_stripe,
+  api_key: System.get_env("STRIPE_SECRET_KEY"),
+  signing_secret: System.get_env("STRIPE_WEBHOOK_SECRET")

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