feat: add Letzshop bidirectional order integration

Add complete Letzshop marketplace integration with:
- GraphQL client for order import and fulfillment operations
- Encrypted credential storage per vendor (Fernet encryption)
- Admin and vendor API endpoints for credentials management
- Order import, confirmation, rejection, and tracking
- Fulfillment queue and sync logging
- Comprehensive documentation and test coverage

New files:
- app/services/letzshop/ - GraphQL client and services
- app/utils/encryption.py - Fernet encryption utility
- models/database/letzshop.py - Database models
- models/schema/letzshop.py - Pydantic schemas
- app/api/v1/admin/letzshop.py - Admin API endpoints
- app/api/v1/vendor/letzshop.py - Vendor API endpoints
- docs/guides/letzshop-order-integration.md - Documentation

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

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

models/database/__init__.py

@@ -28,6 +28,12 @@ from .marketplace_product import (
 )
 from .marketplace_product_translation import MarketplaceProductTranslation
 from .order import Order, OrderItem
+from .letzshop import (
+    LetzshopFulfillmentQueue,
+    LetzshopOrder,
+    LetzshopSyncLog,
+    VendorLetzshopCredentials,
+)
 from .product import Product
 from .product_translation import ProductTranslation
 from .user import User
@@ -82,4 +88,9 @@ __all__ = [
     # Orders
     "Order",
     "OrderItem",
+    # Letzshop Integration
+    "VendorLetzshopCredentials",
+    "LetzshopOrder",
+    "LetzshopFulfillmentQueue",
+    "LetzshopSyncLog",
 ]