docs: add Vendor Operations expansion migration plan

- Create comprehensive migration plan for Vendor Operations expansion
- Document planned phases: Inventory, Orders, Shipping management
- Include database schema designs and API endpoint specifications
- Move Customers from Platform Administration to Vendor Operations
- Update mkdocs navigation with new migration document

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

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

app/templates/admin/partials/sidebar.html

@@ -72,7 +72,6 @@
         {{ menu_item('companies', '/admin/companies', 'office-building', 'Companies') }}
         {{ menu_item('vendors', '/admin/vendors', 'shopping-bag', 'Vendors') }}
         {{ menu_item('users', '/admin/users', 'users', 'Users') }}
-        {{ menu_item('customers', '/admin/customers', 'user-group', 'Customers') }}
     {% endcall %}
 
     <!-- Vendor Operations Section -->
@@ -80,6 +79,7 @@
     {% call section_content('vendorOps') %}
         {{ menu_item('marketplace-products', '/admin/marketplace-products', 'database', 'Marketplace Products') }}
         {{ menu_item('vendor-products', '/admin/vendor-products', 'cube', 'Vendor Products') }}
+        {{ menu_item('customers', '/admin/customers', 'user-group', 'Customers') }}
         {# Future items - uncomment when implemented:
         {{ menu_item('inventory', '/admin/inventory', 'archive', 'Inventory') }}
         {{ menu_item('orders', '/admin/orders', 'clipboard-list', 'Orders') }}