Restore a branch to a point in its history

The worker scans every live database of the branch, as `cloud_admin`, for code owned by a non-Admin role that can run in someone else's session (an Admin's included): triggers whose function or table such a role owns, rules on its relations, and its functions that are SECURITY DEFINER or reached from a policy, a default or generated column, a check, an index, a view, a trigger, a rule, a range type or another such function. Each item has a stable `key`, its `reasons`, and `new` when a list was acknowledged that didn't hold it. `hash` is what protecting (`review_hash`), acknowledging and fixing confirm. Needs the branch's compute running.