refactor(routes): move storefront routes to their modules

Phase 4 of storefront restructure plan - move API routes from legacy
app/api/v1/storefront/ to their respective modules:

- customers: auth, profile, addresses routes combined into storefront.py
- orders: order history viewing routes
- checkout: order placement (place_order endpoint)
- messaging: customer messaging routes

Updated app/api/v1/storefront/__init__.py to import from modules:
- cart_router from app.modules.cart
- catalog_router from app.modules.catalog
- checkout_router from app.modules.checkout
- customers_router from app.modules.customers
- orders_router from app.modules.orders
- messaging_router from app.modules.messaging

Legacy route files in app/api/v1/storefront/ can now be deleted
in Phase 6.

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

docs/proposals/PLAN_storefront-module-restructure.md

@@ -313,7 +313,7 @@ After migrated to `app/modules/cart/services/cart_service.py`.
 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
+4. **Phase 4** - Move routes to modules ✅ COMPLETE
 5. **Phase 5** - Fix direct model imports
 6. **Phase 6** - Delete legacy files
 7. **Phase 7** - Update documentation