Jellyfin does not cascade a user delete to the Device rows redemption
creates, and DeviceManager throws for the whole listing when one device's
user is missing, so a single leftover guest 404s the dashboard devices page.
Deletes the guest's devices before the user, and sweeps devices this plugin
created whose user is already gone at startup for links torn down by older
builds.
Includes fix for redemption failing with DbUpdateConcurrencyException:
change the guest password before applying the user policy, since
UpdatePolicyAsync bumps the user's EF concurrency token and a stale
instance then breaks ChangePassword.
Francois CB
Franciskid