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>

PROGRESS.md

@@ -7,19 +7,22 @@
 **Working:**
 - Theme editor with 8 presets, instant switching, full customization
 - Image optimization pipeline (AVIF/WebP/JPEG responsive variants)
-- Shop pages (home, collections, products, cart, about, contact)
+- Shop pages (home, collections, products, cart, about, contact, error)
 - Mobile-first design with bottom navigation
 - 100% PageSpeed score
 - Variant selector with color swatches and size buttons
+- Session-based cart with real variant data (add/remove/quantity, cross-tab sync)
+- Cart drawer and cart page with hydrated product info
+- Search modal with keyboard shortcut
+- Demo content polished and ready for production
 
-**In Progress:**
-- Session-based cart
+**Next Up:**
+- Orders & Fulfillment (Printify submission)
 
 ## Next Up
 
-1. **Session-based Cart** - Real cart with actual variants
-2. **Stripe Checkout Integration** - Payment processing
-3. **Orders & Fulfillment** - Submit orders to Printify
+1. **Orders & Fulfillment** - Submit orders to Printify after payment
+2. **Email Notifications** - Order confirmation emails
 
 ---
 
@@ -87,19 +90,37 @@ See: [docs/plans/products-context.md](docs/plans/products-context.md) for implem
 See: [docs/plans/printify-integration-research.md](docs/plans/printify-integration-research.md) for API research & risk analysis
 
 ### Cart & Checkout
-**Status:** Planned
+**Status:** In Progress
 
-- [ ] Session-based cart module
-- [ ] Cart LiveView with real variants
-- [ ] Stripe Checkout integration
-- [ ] Order creation and persistence
+- [x] Cart drawer component with slide-over panel (f244a42)
+- [x] Cart page with item list and order summary (f244a42)
+- [x] Shared CartHook for cross-page cart events (f244a42)
+- [x] CartPersist JS hook for localStorage backup
+- [x] Add-to-cart with flash status feedback
+- [x] Cart item links to product pages
+- [x] Session-based cart with real variants (f244a42)
+  - Cart stores {variant_id, qty} tuples in session
+  - Hydrates with real product data via Products context
+  - Cross-tab sync via PubSub, session persistence via CartController API
+- [x] Stripe Checkout integration (stripity_stripe ~> 3.2)
+  - Stripe-hosted Checkout with redirect flow
+  - Webhook handler for checkout.session.completed/expired
+  - Signature verification via CacheRawBody + construct_event
+  - Shipping address collection during checkout
+- [x] Order/OrderItem schemas and context
+  - Order number format: SS-YYMMDD-XXXX
+  - Payment status tracking (pending → paid/failed)
+  - Price snapshots in OrderItem (protects against changes)
+  - Idempotent webhook processing
+- [x] Checkout success page with real-time PubSub updates
+- [x] Cart clearing after successful payment
 
 See: [ROADMAP.md](ROADMAP.md) for design notes
 
 ### Orders & Fulfillment
 **Status:** Planned
 
-- [ ] Orders context with schemas
+- [x] Orders context with schemas
 - [ ] Order submission to Printify
 - [ ] Order status tracking
 - [ ] Customer notifications
@@ -119,6 +140,8 @@ See: [docs/plans/page-builder.md](docs/plans/page-builder.md) for design
 
 | Feature | Commit | Notes |
 |---------|--------|-------|
+| Demo content & link fixes | cff2170 | Broken links, placeholder text, responsive about image |
+| Cart UI infrastructure | f244a42 | Cart drawer, cart page, CartHook, CartPersist |
 | Variant selector | 880e7a2 | Color swatches, size buttons, price updates |
 | Product image download | 1b49b47 | PageSpeed 100% with local images |
 | Wire shop to real data | c818d03 | PreviewData uses Products context |