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/api/v1/vendor/letzshop.py

@@ -18,7 +18,12 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
-from app.exceptions import ResourceNotFoundException, ValidationException
+from app.exceptions import (
+    OrderHasUnresolvedExceptionsException,
+    ResourceNotFoundException,
+    ValidationException,
+)
+from app.services.order_item_exception_service import order_item_exception_service
 from app.services.letzshop import (
     CredentialsNotFoundError,
     LetzshopClientError,
@@ -434,7 +439,12 @@ def confirm_order(
     current_user: User = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
-    """Confirm inventory units for a Letzshop order."""
+    """
+    Confirm inventory units for a Letzshop order.
+
+    Raises:
+        OrderHasUnresolvedExceptionsException: If order has unresolved product exceptions
+    """
     vendor_id = current_user.token_vendor_id
     order_service = get_order_service(db)
     creds_service = get_credentials_service(db)
@@ -444,6 +454,13 @@ def confirm_order(
     except OrderNotFoundError:
         raise ResourceNotFoundException("LetzshopOrder", str(order_id))
 
+    # Check for unresolved exceptions (blocks confirmation)
+    unresolved_count = order_item_exception_service.get_unresolved_exception_count(
+        db, order_id
+    )
+    if unresolved_count > 0:
+        raise OrderHasUnresolvedExceptionsException(order_id, unresolved_count)
+
     # Get inventory unit IDs from request or order
     if confirm_request and confirm_request.inventory_unit_ids:
         inventory_unit_ids = confirm_request.inventory_unit_ids