docs: update documentation for cart implementation

Updated API reference, error handling, and architecture docs to reflect
the new persistent cart system with database storage.

API Reference Updates (shop-api-reference.md):
- Updated last modified date to 2025-11-23
- Updated cart response schemas to match CartResponse/CartItemResponse models
- Added detailed schema tables for all cart endpoints
- Documented all cart exceptions with examples (CART_ITEM_NOT_FOUND, INSUFFICIENT_INVENTORY_FOR_CART, etc.)
- Added implementation notes about cart persistence and duplicate prevention
- Updated all endpoint documentation with proper request/response schemas
- Added CartOperationResponse and ClearCartResponse documentation

Error Handling Updates (error-handling.md):
- Added Shopping Cart Exceptions section with 6 cart-specific exceptions
- Added Product Exceptions section
- Added Inventory Exceptions section
- Updated error response format to show structured WizamartException format
- Added examples with error_code, message, status_code, and details fields
- Documented the difference between structured and generic error formats

Architecture Updates (overview.md):
- Added cart_items table to database schema diagram
- Documented session-based shopping cart in data model hierarchy

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/architecture/overview.md

@@ -170,6 +170,8 @@ graph TB
          │                 ├─── product_images
          │                 └─── product_categories
          │
+         ├─── cart_items (session-based shopping cart)
+         │
          ├─── orders ──────┬─── order_items
          │                 └─── order_status_history
          │