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>

mkdocs.yml

@@ -136,6 +136,7 @@ nav:
     - Implementation Plans:
       - Admin Inventory Management: implementation/inventory-admin-migration.md
       - Letzshop Order Import: implementation/letzshop-order-import-improvements.md
+      - Order Item Exceptions: implementation/order-item-exceptions.md
       - Unified Order View: implementation/unified-order-view.md
     - Seed Scripts Audit: development/seed-scripts-audit.md
     - Database Seeder: