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>

app/modules/customers/schemas/__init__.py

@@ -9,9 +9,11 @@ Usage:
         CustomerRegister,
         CustomerUpdate,
         CustomerResponse,
+        CustomerContext,
     )
 """
 
+from app.modules.customers.schemas.context import CustomerContext
 from app.modules.customers.schemas.customer import (
     # Registration & Authentication
     CustomerRegister,
@@ -41,6 +43,8 @@ from app.modules.customers.schemas.customer import (
 )
 
 __all__ = [
+    # Context (for dependency injection)
+    "CustomerContext",
     # Registration & Authentication
     "CustomerRegister",
     "CustomerUpdate",