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/api/deps.py

@@ -716,7 +716,7 @@ def get_current_vendor_api(
 
 def _validate_customer_token(token: str, request: Request, db: Session):
     """
-    Validate customer JWT token and return Customer object.
+    Validate customer JWT token and return CustomerContext schema.
 
     Validates:
     1. Token signature and expiration
@@ -730,7 +730,7 @@ def _validate_customer_token(token: str, request: Request, db: Session):
         db: Database session
 
     Returns:
-        Customer: Authenticated customer object
+        CustomerContext: Authenticated customer context schema
 
     Raises:
         InvalidTokenException: If token is invalid or expired
@@ -741,6 +741,7 @@ def _validate_customer_token(token: str, request: Request, db: Session):
     from jose import JWTError, jwt
 
     from app.modules.customers.models.customer import Customer
+    from app.modules.customers.schemas import CustomerContext
 
     # Decode and validate customer JWT token
     try:
@@ -800,7 +801,8 @@ def _validate_customer_token(token: str, request: Request, db: Session):
 
     logger.debug(f"Customer authenticated: {customer.email} (ID: {customer.id})")
 
-    return customer
+    # Return CustomerContext schema instead of database model
+    return CustomerContext.from_db_model(customer)
 
 
 def get_current_customer_from_cookie_or_header(
@@ -828,7 +830,7 @@ def get_current_customer_from_cookie_or_header(
         db: Database session
 
     Returns:
-        Customer: Authenticated customer object
+        CustomerContext: Authenticated customer context schema
 
     Raises:
         InvalidTokenException: If no token or invalid token
@@ -862,7 +864,7 @@ def get_current_customer_api(
         db: Database session
 
     Returns:
-        Customer: Authenticated customer object
+        CustomerContext: Authenticated customer context schema
 
     Raises:
         InvalidTokenException: If no token or invalid token
@@ -1327,7 +1329,7 @@ def get_current_customer_optional(
         db: Database session
 
     Returns:
-        Customer: Authenticated customer if valid token exists
+        CustomerContext: Authenticated customer context if valid token exists
         None: If no token, invalid token, or vendor mismatch
     """
     token, source = _get_token_from_request(