refactor(arch): use CustomerContext schema for dependency injection

Phase 5 of storefront restructure plan - fix direct model imports in
API routes by using schemas for dependency injection.

Created CustomerContext schema:
- Lightweight Pydantic model for customer data in API routes
- Populated from Customer DB model in auth dependency
- Contains all fields needed by storefront routes
- Includes from_db_model() factory method

Updated app/api/deps.py:
- _validate_customer_token now returns CustomerContext instead of Customer
- Updated docstrings for all customer auth functions

Updated module storefront routes:
- customers: Uses CustomerContext for profile/address endpoints
- orders: Uses CustomerContext for order history endpoints
- checkout: Uses CustomerContext for order placement
- messaging: Uses CustomerContext for messaging endpoints

This enforces the layered architecture (Routes → Services → Models)
by ensuring API routes never import database models directly.

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

docs/proposals/PLAN_storefront-module-restructure.md

@@ -314,7 +314,7 @@ After migrated to `app/modules/cart/services/cart_service.py`.
 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 ✅ COMPLETE
-5. **Phase 5** - Fix direct model imports
+5. **Phase 5** - Fix direct model imports ✅ COMPLETE
 6. **Phase 6** - Delete legacy files
 7. **Phase 7** - Update documentation