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/api/index.md

@@ -26,8 +26,8 @@ All API endpoints are versioned using URL path versioning:
 
 ## Endpoint Categories
 
-### Shop API (`/shop/`) - Customer-Facing
-Multi-tenant shop endpoints with automatic vendor context from middleware:
+### Storefront API (`/storefront/`) - Customer-Facing
+Multi-tenant storefront endpoints with automatic vendor context from middleware:
 - **Products**: Browse catalog, search, product details
 - **Cart**: Shopping cart operations (session-based)
 - **Orders**: Order placement and history (authenticated)