Set CRM order status to PACKED after Checkbox accepts the receipt

- ExoCrmClient.set_status: live SetStatus needs {Orders: [id], Status} and
  replies per order, unlike the documented {ID, Status}
- receipts.crm_status_set_at (migration 0006); set right after creation,
  retried by cron, row-locked to avoid a repeat PACKED overwriting a newer status
- CRM errors under capitalized 'Errors' and non-JSON replies are reported

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/crm/client.py

@@ -13,3 +13,5 @@ class CrmError(Exception):
 
 class CrmClient(Protocol):
     async def get_orders(self, *, status: str) -> list[OrderOut]: ...
+
+    async def set_status(self, *, order_id: str, status: str) -> None: ...