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/__init__.py

@@ -38,6 +38,7 @@ from . import (
     marketplace,
     monitoring,
     notifications,
+    order_item_exceptions,
     orders,
     products,
     settings,
@@ -115,6 +116,11 @@ router.include_router(inventory.router, tags=["admin-inventory"])
 # Include order management endpoints
 router.include_router(orders.router, tags=["admin-orders"])
 
+# Include order item exception management endpoints
+router.include_router(
+    order_item_exceptions.router, tags=["admin-order-exceptions"]
+)
+
 
 # ============================================================================
 # Marketplace & Imports