docs: update documentation for storefront module restructure

- Rename shop-api-reference.md to storefront-api-reference.md
- Update all /api/v1/shop paths to /api/v1/storefront
- Add cart, catalog, checkout modules to module-system.md
- Update API index to reference Storefront API
- Mark Phase 7 as complete in PLAN document

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

docs/architecture/module-system.md

@@ -83,12 +83,15 @@ Core modules are **always enabled** and cannot be disabled. They provide fundame
 | `cms` | Content pages, media library, themes | Content management |
 | `customers` | Customer database, profiles, segmentation | Customer data management |
 
-### Optional Modules (7)
+### Optional Modules (10)
 
 Optional modules can be **enabled or disabled per platform**. They provide additional functionality that may not be needed by all platforms.
 
 | Module | Dependencies | Description |
 |--------|--------------|-------------|
+| `cart` | - | Shopping cart management, session-based carts |
+| `catalog` | - | Customer-facing product browsing |
+| `checkout` | `cart`, `orders`, `payments` | Cart-to-order conversion, checkout flow |
 | `payments` | - | Payment gateway integrations (Stripe, PayPal, etc.) |
 | `billing` | `payments` | Platform subscriptions, vendor invoices |
 | `inventory` | - | Stock management, locations |
@@ -283,13 +286,14 @@ The Framework Layer provides infrastructure that modules depend on. These are **
 Modules can depend on other modules. When enabling a module, its dependencies are automatically enabled.
 
 ```
-                    payments
-                   ↙        ↘
-              billing        orders
-
-              inventory
-                  ↓
-             marketplace
+                         payments
+                        ↙        ↘
+                   billing        orders ←──┐
+                                     ↑      │
+              inventory              │      │
+                  ↓                cart     │
+             marketplace               ↘    │
+                                      checkout
 ```
 
 **Dependency Rules:**