feat: add automatic stock synchronization for orders

Implements order-inventory integration that automatically manages stock
when order status changes:
- processing: reserves inventory for order items
- shipped: fulfills (deducts) from stock
- cancelled: releases reserved inventory

Creates OrderInventoryService to orchestrate operations between
OrderService and InventoryService. Placeholder products (unmatched
Letzshop items) are skipped. Inventory errors are logged but don't
block order status updates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -167,6 +167,7 @@ nav:
       - VAT Invoice Feature: implementation/vat-invoice-feature.md
       - OMS Feature Plan: implementation/oms-feature-plan.md
       - Vendor Frontend Parity: implementation/vendor-frontend-parity-plan.md
+      - Stock Management Integration: implementation/stock-management-integration.md
 
     # --- Testing ---
     - Testing: