docs: add messaging system implementation documentation

- Add comprehensive messaging-system.md documentation
- Document database models, services, API endpoints
- Document admin, vendor, and shop interfaces
- Document future enhancements (email notifications, WebSocket)
- Add to mkdocs.yml navigation

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

mkdocs.yml

@@ -136,8 +136,10 @@ nav:
       - Vendor Operations Expansion: development/migration/vendor-operations-expansion.md
     - Implementation Plans:
       - Admin Inventory Management: implementation/inventory-admin-migration.md
+      - Admin Notification System: implementation/admin-notification-system.md
       - Letzshop Order Import: implementation/letzshop-order-import-improvements.md
       - Letzshop Jobs & Tables: implementation/letzshop-jobs-improvements.md
+      - Messaging System: implementation/messaging-system.md
       - Order Item Exceptions: implementation/order-item-exceptions.md
       - Product Suppliers Table: implementation/product-suppliers-table.md
       - Unified Order View: implementation/unified-order-view.md