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/services/letzshop/__init__.py

@@ -0,0 +1,45 @@
+# app/services/letzshop/__init__.py
+"""
+Letzshop marketplace integration services.
+
+Provides:
+- GraphQL client for API communication
+- Credential management service
+- Order import service
+- Fulfillment sync service
+"""
+
+from .client import (
+    LetzshopAPIError,
+    LetzshopAuthError,
+    LetzshopClient,
+    LetzshopClientError,
+    LetzshopConnectionError,
+)
+from .credentials import (
+    CredentialsError,
+    CredentialsNotFoundError,
+    LetzshopCredentialsService,
+)
+from .order_service import (
+    LetzshopOrderService,
+    OrderNotFoundError,
+    VendorNotFoundError,
+)
+
+__all__ = [
+    # Client
+    "LetzshopClient",
+    "LetzshopClientError",
+    "LetzshopAuthError",
+    "LetzshopAPIError",
+    "LetzshopConnectionError",
+    # Credentials
+    "LetzshopCredentialsService",
+    "CredentialsError",
+    "CredentialsNotFoundError",
+    # Order Service
+    "LetzshopOrderService",
+    "OrderNotFoundError",
+    "VendorNotFoundError",
+]