Read-only Vikunja users can delete project background images via broken object-level authorization

Vikunja is an open-source self-hosted task management platform. Starting in version 0.20.2 and prior to version 2.2.0, the `DELETE /api/v1/projects/:project/background` endpoint checks `CanRead` permission instead of `CanUpdate`, allowing any user with read-only access to a project to permanently delete its background image. Version 2.2.0 fixes the issue.