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>

app/exceptions/__init__.py

@@ -135,6 +135,14 @@ from .order import (
     OrderValidationException,
 )
 
+# Order item exception exceptions
+from .order_item_exception import (
+    ExceptionAlreadyResolvedException,
+    InvalidProductForExceptionException,
+    OrderHasUnresolvedExceptionsException,
+    OrderItemExceptionNotFoundException,
+)
+
 # Product exceptions
 from .product import (
     CannotDeleteProductWithInventoryException,
@@ -308,6 +316,11 @@ __all__ = [
     "OrderValidationException",
     "InvalidOrderStatusException",
     "OrderCannotBeCancelledException",
+    # Order item exception exceptions
+    "OrderItemExceptionNotFoundException",
+    "OrderHasUnresolvedExceptionsException",
+    "ExceptionAlreadyResolvedException",
+    "InvalidProductForExceptionException",
     # Cart exceptions
     "CartItemNotFoundException",
     "EmptyCartException",