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>

static/admin/js/init-alpine.js

@@ -63,10 +63,10 @@ function data() {
     companies: 'platformAdmin',
     vendors: 'platformAdmin',
     users: 'platformAdmin',
-    customers: 'platformAdmin',
-    // Vendor Operations (Products, Inventory, Orders, Shipping)
+    // Vendor Operations (Products, Customers, Inventory, Orders, Shipping)
     'marketplace-products': 'vendorOps',
     'vendor-products': 'vendorOps',
+    customers: 'vendorOps',
     // Future: inventory, orders, shipping will map to 'vendorOps'
     // Marketplace
     'marketplace-letzshop': 'marketplace',