feat(modules): create cart, catalog, and checkout e-commerce modules

Phase 3 of storefront restructure plan - create dedicated modules for
e-commerce functionality:

- cart: Shopping cart management with storefront API routes
  - CartItem model with cents-based pricing
  - CartService for cart operations
  - Storefront routes for cart CRUD operations

- catalog: Product catalog browsing for customers
  - CatalogService for public product queries
  - Storefront routes for product listing/search/details

- checkout: Order creation from cart (placeholder)
  - CheckoutService stub for future cart-to-order conversion
  - Schemas for checkout flow

These modules separate e-commerce concerns from core platform
concerns (customer auth), enabling non-commerce platforms.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/proposals/PLAN_storefront-module-restructure.md

@@ -310,9 +310,9 @@ After migrated to `app/modules/cart/services/cart_service.py`.
 
 ## Execution Order
 
-1. **Phase 1** - Add architecture rule (enables detection)
-2. **Phase 2** - Rename shop → storefront (terminology)
-3. **Phase 3** - Create new modules (cart, checkout, catalog)
+1. **Phase 1** - Add architecture rule (enables detection) ✅ COMPLETE
+2. **Phase 2** - Rename shop → storefront (terminology) ✅ COMPLETE
+3. **Phase 3** - Create new modules (cart, checkout, catalog) ✅ COMPLETE
 4. **Phase 4** - Move routes to modules
 5. **Phase 5** - Fix direct model imports
 6. **Phase 6** - Delete legacy files