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/cart/definition.py

@@ -0,0 +1,25 @@
+# app/modules/cart/definition.py
+"""
+Cart module definition.
+
+This module provides shopping cart functionality for customer storefronts.
+It is session-based and does not require customer authentication.
+"""
+
+from app.modules.core.module_registry import ModuleDefinition
+
+module = ModuleDefinition(
+    code="cart",
+    name="Shopping Cart",
+    description="Session-based shopping cart for storefronts",
+    version="1.0.0",
+    is_self_contained=True,
+    dependencies=["inventory"],  # Checks inventory availability
+    provides_models=True,
+    provides_schemas=True,
+    provides_services=True,
+    provides_api_routes=True,
+    provides_page_routes=False,
+    provides_admin_ui=False,
+    provides_vendor_ui=False,
+)