Fix scheduling due to missing device id

[Arnei]

Fixes #1515.

The function that creates the scheduling request for the backend was looking for the device id in the wrong place.

How to test

Check if scheduling an event works again with this PR. You can create a dummy capture agent if none is available with the /capture-admin/agents/{name} endpoint.

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-1517

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-1517

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/1517/2026-03-05_08-59-20/ .
It might take a few minutes for it to become available.

[gregorydlogan]

Normally I would wait for a technical meeting review pass to get this in, but it's directly affecting test infra and likely adopters.