Preview what a delete would destroy

For a client’s own delete confirmation screen: the rows that would go, the links that would be broken, and the sentences to show. When plan.blocked is true the delete will be refused and the sentences are the refusal — render them and disable the button rather than letting the user find out on the DELETE.

Gated on the same permission the delete itself requires, so a caller cannot use the preview to learn about rows they could not delete. A row belonging to another organisation answers 404, the same as one that does not exist. Works with an API token.