feat: add unified admin Marketplace Letzshop page

- Add new Marketplace section in admin sidebar with Letzshop sub-item
- Remove old Import and Letzshop Orders items from Product Catalog
- Create unified Letzshop management page with 3 tabs:
  - Products tab: Import/Export functionality
  - Orders tab: Order management with confirm/reject/tracking
  - Settings tab: API credentials and CSV URLs
- Add unified jobs table showing imports, exports, and order syncs
- Implement vendor autocomplete using Tom Select library (CDN + fallback)
- Add /vendors/{vendor_id}/jobs API endpoint for unified job listing
- Move database queries to service layer (LetzshopOrderService)
- Add LetzshopJobItem and LetzshopJobsListResponse schemas
- Include Tom Select CSS/JS assets as local fallback

🤖 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

@@ -80,8 +80,12 @@
     {% call section_content('productCatalog') %}
         {{ menu_item('marketplace-products', '/admin/marketplace-products', 'database', 'Marketplace Products') }}
         {{ menu_item('vendor-products', '/admin/vendor-products', 'cube', 'Vendor Products') }}
-        {{ menu_item('marketplace', '/admin/marketplace', 'cloud-download', 'Import') }}
-        {{ menu_item('letzshop', '/admin/letzshop', 'shopping-cart', 'Letzshop Orders') }}
+    {% endcall %}
+
+    <!-- Marketplace Section -->
+    {{ section_header('Marketplace', 'marketplace') }}
+    {% call section_content('marketplace') %}
+        {{ menu_item('marketplace-letzshop', '/admin/marketplace/letzshop', 'shopping-cart', 'Letzshop') }}
     {% endcall %}
 
     <!-- Content Management Section -->