Health enpoint should check if anti forgery files can be read/written

### Description

If mounting the filesystem with the AntiForgery keys fails and the files are not read/write able the application will fail to handle requests.
We should include a default check on the `/health` endpoint the is used by the liveness and readiness probes in k8s to verify that the app is ready to serve traffic.

### Additional Information

_No response_

Start at the /health endpoint used by Kubernetes liveness and readiness probes, then trace how the AntiForgery key files are configured. Done means the default health check reports the application as not ready when those files cannot be read or written, and remains healthy when they can; add or update relevant tests if the repository provides them.

Written by the indexing model from the issue text.