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>

app/modules/checkout/__init__.py

@@ -0,0 +1,13 @@
+# app/modules/checkout/__init__.py
+"""
+Checkout Module
+
+Provides checkout and order creation functionality for storefronts, including:
+- Cart to order conversion
+- Shipping address handling
+- Payment method selection
+- Order confirmation
+
+Note: This module is a placeholder for future checkout functionality.
+Currently, order creation is handled directly through the orders module.
+"""