fix: commit orders immediately after creation during historical import

Orders were being flushed but only committed every 10 orders via the
progress callback. With slow imports (~1.5 min/order), no commits
happened before the first 10 orders, causing data loss if the import
failed or was interrupted.

Changes:
- Add db.commit() after each successful order creation
- Add db.rollback() on order creation failure to prevent corrupt state
- Add db.commit() after order updates

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

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

app/services/letzshop/order_service.py

@@ -733,6 +733,7 @@ class LetzshopOrderService:
                             pass
 
                 if needs_update:
+                    self.db.commit()  # Commit update immediately
                     stats["updated"] += 1
                 else:
                     stats["skipped"] += 1
@@ -740,8 +741,10 @@ class LetzshopOrderService:
                 # Create new order using unified service
                 try:
                     self.create_order(vendor_id, shipment)
+                    self.db.commit()  # Commit each order immediately
                     stats["imported"] += 1
                 except Exception as e:
+                    self.db.rollback()  # Rollback failed order
                     stats["errors"] += 1
                     stats["error_messages"].append(
                         f"Shipment {shipment_id}: {str(e)}"