feat: add Letzshop vendor directory with sync and admin management

- Add LetzshopVendorCache model to store cached vendor data from Letzshop API
- Create LetzshopVendorSyncService for syncing vendor directory
- Add Celery task for background vendor sync
- Create admin page at /admin/letzshop/vendor-directory with:
  - Stats dashboard (total, claimed, unclaimed vendors)
  - Searchable/filterable vendor list
  - "Sync Now" button to trigger sync
  - Ability to create platform vendors from Letzshop cache
- Add API endpoints for vendor directory management
- Add Pydantic schemas for API responses

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

app/services/letzshop/__init__.py

@@ -7,6 +7,7 @@ Provides:
 - Credential management service
 - Order import service
 - Fulfillment sync service
+- Vendor directory sync service
 """
 
 from .client_service import (
@@ -26,6 +27,10 @@ from .order_service import (
     OrderNotFoundError,
     VendorNotFoundError,
 )
+from .vendor_sync_service import (
+    LetzshopVendorSyncService,
+    get_vendor_sync_service,
+)
 
 __all__ = [
     # Client
@@ -42,4 +47,7 @@ __all__ = [
     "LetzshopOrderService",
     "OrderNotFoundError",
     "VendorNotFoundError",
+    # Vendor Sync Service
+    "LetzshopVendorSyncService",
+    "get_vendor_sync_service",
 ]