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/proposals/PLAN_storefront-module-restructure.md

@@ -316,7 +316,7 @@ After migrated to `app/modules/cart/services/cart_service.py`.
 4. **Phase 4** - Move routes to modules ✅ COMPLETE
 5. **Phase 5** - Fix direct model imports ✅ COMPLETE
 6. **Phase 6** - Delete legacy files ✅ COMPLETE
-7. **Phase 7** - Update documentation
+7. **Phase 7** - Update documentation ✅ COMPLETE
 
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