Ensure REST API handles SSH key expiry todos

What does this MR do and why?

Ensure REST API handles SSH key expiry todos

This ensures the SSH todo type properly maps to the SSHKey API entity to prevent 500 errors when hitting the REST todos API.

Changelog: fixed

References

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How to set up and validate locally

Create an SSH expiry to-do

• Create an SSH key at http://gdk.test:3000/-/user_settings/ssh_keys
• Set its expires_at to today via the rails console: Key.last.update(expires_at: 1.hour.ago)
• Don't set an earlier date here, as the following background job only notifies about the keys that expired today.
• Still in the console, trigger the job that would otherwise run only twice a day: SshKeys::ExpiredNotificationWorker.new.perform

Hit the REST API

• Navigate to http://gdk.test:3000/api/v4/todos?per_page=100
  • On master, this should throw a 500 error.
  • On this branch, the endpoint should load without issues.

Related to #510792