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>

app/api/v1/vendor/__init__.py

@@ -21,6 +21,7 @@ from . import (
     dashboard,
     info,
     inventory,
+    letzshop,
     marketplace,
     media,
     notifications,
@@ -59,6 +60,7 @@ router.include_router(customers.router, tags=["vendor-customers"])
 router.include_router(team.router, tags=["vendor-team"])
 router.include_router(inventory.router, tags=["vendor-inventory"])
 router.include_router(marketplace.router, tags=["vendor-marketplace"])
+router.include_router(letzshop.router, tags=["vendor-letzshop"])
 
 # Services (with prefixes: /payments/*, /media/*, etc.)
 router.include_router(payments.router, tags=["vendor-payments"])