feat(#3634706): expire registered preview targets and check frontends from the browser

Adds a PATCH on the targets endpoint that registers one target and records when it was seen. A target that has not been registered for a day moves to an Inactive group in the Frontend picker, and cron deletes it after 30 days. Targets added by hand never expire. When the preview opens, the browser also asks each frontend's origin for a response, and marks one that does not answer