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/admin/letzshop.py

@@ -16,7 +16,12 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_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,
@@ -783,6 +788,9 @@ def confirm_order(
     Confirm all inventory units for a Letzshop order.
 
     Sends confirmInventoryUnits mutation with isAvailable=true for all items.
+
+    Raises:
+        OrderHasUnresolvedExceptionsException: If order has unresolved product exceptions
     """
     order_service = get_order_service(db)
     creds_service = get_credentials_service(db)
@@ -792,6 +800,13 @@ def confirm_order(
     except OrderNotFoundError:
         raise ResourceNotFoundException("Order", 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 order items
     items = order_service.get_order_items(order)
     if not items:
@@ -933,6 +948,9 @@ def confirm_single_item(
     Confirm a single inventory unit in an order.
 
     Sends confirmInventoryUnits mutation with isAvailable=true for one item.
+
+    Raises:
+        OrderHasUnresolvedExceptionsException: If the specific item has an unresolved exception
     """
     order_service = get_order_service(db)
     creds_service = get_credentials_service(db)
@@ -942,6 +960,15 @@ def confirm_single_item(
     except OrderNotFoundError:
         raise ResourceNotFoundException("Order", str(order_id))
 
+    # Check if this specific item has an unresolved exception
+    # Find the order item by external_item_id
+    item = next(
+        (i for i in order.items if i.external_item_id == item_id),
+        None
+    )
+    if item and item.needs_product_match:
+        raise OrderHasUnresolvedExceptionsException(order_id, 1)
+
     try:
         with creds_service.create_client(vendor_id) as client:
             result = client.confirm_inventory_units([item_id])