feat: add order item exception system for graceful product matching

Replaces the "fail on missing product" behavior with graceful handling:
- Orders import even when products aren't found by GTIN
- Unmatched items link to a per-vendor placeholder product
- Exceptions tracked in order_item_exceptions table for QC resolution
- Order confirmation blocked until exceptions are resolved
- Auto-matching when products are imported via catalog sync

New files:
- OrderItemException model and migration
- OrderItemExceptionService with CRUD and resolution logic
- Admin and vendor API endpoints for exception management
- Domain exceptions for error handling

Modified:
- OrderItem: added needs_product_match flag and exception relationship
- OrderService: graceful handling with placeholder products
- MarketplaceProductService: auto-match on product import
- Letzshop confirm endpoints: blocking check for unresolved exceptions

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

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

models/database/__init__.py

@@ -33,6 +33,7 @@ from .marketplace_product import (
 )
 from .marketplace_product_translation import MarketplaceProductTranslation
 from .order import Order, OrderItem
+from .order_item_exception import OrderItemException
 from .product import Product
 from .product_translation import ProductTranslation
 from .test_run import TestCollection, TestResult, TestRun
@@ -89,6 +90,7 @@ __all__ = [
     # Orders
     "Order",
     "OrderItem",
+    "OrderItemException",
     # Letzshop Integration
     "VendorLetzshopCredentials",
     "LetzshopFulfillmentQueue",