refactor(cleanup): delete legacy storefront routes, convert cart to re-exports

Phase 6 of storefront restructure plan - delete legacy files and convert
remaining cart files to re-exports.

Deleted legacy storefront route files (now served from modules):
- app/api/v1/storefront/auth.py (→ customers module)
- app/api/v1/storefront/profile.py (→ customers module)
- app/api/v1/storefront/addresses.py (→ customers module)
- app/api/v1/storefront/carts.py (→ cart module)
- app/api/v1/storefront/products.py (→ catalog module)
- app/api/v1/storefront/orders.py (→ orders/checkout modules)
- app/api/v1/storefront/messages.py (→ messaging module)

Converted legacy cart files to re-exports:
- models/schema/cart.py → app.modules.cart.schemas
- models/database/cart.py → app.modules.cart.models
- app/services/cart_service.py → app.modules.cart.services

This reduces API-007 violations from 81 to 69 (remaining violations
are in admin/vendor routes - separate migration effort).

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

docs/proposals/PLAN_storefront-module-restructure.md

@@ -315,7 +315,7 @@ After migrated to `app/modules/cart/services/cart_service.py`.
 3. **Phase 3** - Create new modules (cart, checkout, catalog) ✅ COMPLETE
 4. **Phase 4** - Move routes to modules ✅ COMPLETE
 5. **Phase 5** - Fix direct model imports ✅ COMPLETE
-6. **Phase 6** - Delete legacy files
+6. **Phase 6** - Delete legacy files ✅ COMPLETE
 7. **Phase 7** - Update documentation
 
 ---