docs: add multi-marketplace product architecture documentation

- Add marketplace-integration.md: Full architecture for bidirectional
  marketplace integration (products, orders, fulfillment, inventory sync)
- Add multi-marketplace-product-architecture.md: Detailed product data
  model with translations and vendor override pattern
- Add product-migration-database-changes.md: Database migration plan
  with SQL scripts and model updates
- Update mkdocs.yml with new documentation entries

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

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

mkdocs.yml

@@ -29,6 +29,7 @@ nav:
   # ============================================
   - Architecture:
     - Overview: architecture/overview.md
+    - Marketplace Integration: architecture/marketplace-integration.md
     - Architecture Patterns: architecture/architecture-patterns.md
     - Company-Vendor Management: architecture/company-vendor-management.md
     - Multi-Tenant System: architecture/multi-tenant.md
@@ -127,6 +128,8 @@ nav:
       - Makefile Refactoring: development/migration/makefile-refactoring-complete.md
       - SVC-006 Migration Plan: development/migration/svc-006-migration-plan.md
       - Vendor Contact Inheritance: development/migration/vendor-contact-inheritance.md
+      - Multi-Marketplace Product Architecture: development/migration/multi-marketplace-product-architecture.md
+      - Product Migration Database Changes: development/migration/product-migration-database-changes.md
     - Seed Scripts Audit: development/seed-scripts-audit.md
     - Database Seeder:
       - Documentation: development/database-seeder/database-seeder-documentation.md