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/vendor.py

@@ -96,6 +96,14 @@ class Vendor(Base, TimestampMixin):
         "MarketplaceImportJob", back_populates="vendor"
     )  # Relationship with MarketplaceImportJob model for import jobs related to this vendor
 
+    # Letzshop integration credentials (one-to-one)
+    letzshop_credentials = relationship(
+        "VendorLetzshopCredentials",
+        back_populates="vendor",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
+
     domains = relationship(
         "VendorDomain",
         back_populates="vendor",