fix: Use correct user_id parameter for assignment API

src/views/OrderDetailView.vue

@@ -74,7 +74,7 @@ async function assignSelectedUsers() {
   try {
     // Assign each selected user
     for (const userId of selectedUsers.value) {
-      await api.post(`/orders/${route.params.id}/assign`, { userId })
+      await api.post(`/orders/${route.params.id}/assign`, { user_id: userId })
     }
     showAssignModal.value = false
     await loadOrder()