feat: implement unified order schema with customer/address snapshots

- Update Order model with customer/address snapshot fields
- Add external marketplace references (external_order_id, external_shipment_id)
- Add tracking_provider field for shipping carriers
- Add order_date, confirmed_at timestamps
- Update OrderItem with gtin/gtin_type, external_item_id, item_state
- Remove LetzshopOrder model (orders now go to unified table)
- Update LetzshopFulfillmentQueue to reference orders.id

Design decision: Single orders table for all channels with snapshotted
data preserved at order time for historical accuracy.

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

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

models/database/__init__.py

@@ -21,7 +21,7 @@ from .customer import Customer, CustomerAddress
 from .inventory import Inventory
 from .letzshop import (
     LetzshopFulfillmentQueue,
-    LetzshopOrder,
+    LetzshopHistoricalImportJob,
     LetzshopSyncLog,
     VendorLetzshopCredentials,
 )
@@ -91,7 +91,7 @@ __all__ = [
     "OrderItem",
     # Letzshop Integration
     "VendorLetzshopCredentials",
-    "LetzshopOrder",
     "LetzshopFulfillmentQueue",
     "LetzshopSyncLog",
+    "LetzshopHistoricalImportJob",
 ]