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",

app/exceptions/order_item_exception.py

@@ -0,0 +1,54 @@
+# app/exceptions/order_item_exception.py
+"""Order item exception specific exceptions."""
+
+from .base import BusinessLogicException, ResourceNotFoundException
+
+
+class OrderItemExceptionNotFoundException(ResourceNotFoundException):
+    """Raised when an order item exception is not found."""
+
+    def __init__(self, exception_id: int | str):
+        super().__init__(
+            resource_type="OrderItemException",
+            identifier=str(exception_id),
+            error_code="ORDER_ITEM_EXCEPTION_NOT_FOUND",
+        )
+
+
+class OrderHasUnresolvedExceptionsException(BusinessLogicException):
+    """Raised when trying to confirm an order with unresolved exceptions."""
+
+    def __init__(self, order_id: int, unresolved_count: int):
+        super().__init__(
+            message=(
+                f"Order has {unresolved_count} unresolved product exception(s). "
+                f"Please resolve all exceptions before confirming the order."
+            ),
+            error_code="ORDER_HAS_UNRESOLVED_EXCEPTIONS",
+            details={
+                "order_id": order_id,
+                "unresolved_count": unresolved_count,
+            },
+        )
+
+
+class ExceptionAlreadyResolvedException(BusinessLogicException):
+    """Raised when trying to resolve an already resolved exception."""
+
+    def __init__(self, exception_id: int):
+        super().__init__(
+            message=f"Exception {exception_id} has already been resolved",
+            error_code="EXCEPTION_ALREADY_RESOLVED",
+            details={"exception_id": exception_id},
+        )
+
+
+class InvalidProductForExceptionException(BusinessLogicException):
+    """Raised when the product provided for resolution is invalid."""
+
+    def __init__(self, product_id: int, reason: str):
+        super().__init__(
+            message=f"Cannot use product {product_id} for resolution: {reason}",
+            error_code="INVALID_PRODUCT_FOR_EXCEPTION",
+            details={"product_id": product_id, "reason": reason},
+        )