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/order.py

@@ -10,6 +10,7 @@ from sqlalchemy import (
     String,
     Text,
 )
+from sqlalchemy.dialects.sqlite import JSON
 from sqlalchemy.orm import relationship
 
 from app.core.database import Base
@@ -29,6 +30,15 @@ class Order(Base, TimestampMixin):
 
     order_number = Column(String, nullable=False, unique=True, index=True)
 
+    # Order channel/source
+    channel = Column(
+        String(50), default="direct", index=True
+    )  # direct, letzshop, amazon, etc.
+    external_order_id = Column(
+        String(100), nullable=True, index=True
+    )  # External order reference
+    external_channel_data = Column(JSON, nullable=True)  # Channel-specific metadata
+
     # Order status
     status = Column(String, nullable=False, default="pending", index=True)
     # pending, processing, shipped, delivered, cancelled, refunded