Expire registered preview targets and check frontends from the browser

Problem/Motivation

A frontend can register its preview target over the API, but the target stays on the consumer after the frontend has gone. Local and short-lived environments build up in the Frontend picker, and an editor cannot tell which ones still answer.

A frontend also has to read and replace the whole target list to register, so two frontends on one consumer can overwrite each other.

Proposed resolution

• Add a PATCH on the targets endpoint that registers one target. It replaces the target with the same label or adds a new one, leaves the other targets alone, and records when it was seen.
• Move a target that has not been registered for a day to an Inactive group at the bottom of the Frontend picker. It can still be picked, its key does not change, and the next PATCH moves it back.
• Delete a target on cron after 30 days without a PATCH. Settings can change both limits.
• Never expire a target added by hand, on the consumer form or in settings.php.
• When the preview screen opens, ask each frontend's origin for a response from the editor's browser, with a limit of 3 seconds. A frontend that does not answer shows "(not responding)" and can still be picked. Inactive targets and paths on Drupal's own site are not checked.

Remaining tasks

• Review the merge request.

User interface changes

The Frontend picker gains an Inactive group and a "(not responding)" marker: screenshot.

API changes

• Adds PATCH /api/consumer-node-preview/targets.
• Adds the inactive_after and delete_after keys to $settings['consumer_node_preview'].
• The targets from PreviewTargetsProviderInterface::getAllTargets() gain last_seen and inactive values.

Data model changes

Each item in the preview_targets field gains a last_seen value. There is no update hook, because the module has no release yet.