Get a readable board and caller role

One readable board plus the caller's own role on it. While the workspace is read-only — suspended by the operator, or with a trial that ran out, a canceled subscription or no plan chosen — role is viewer for every member and board writes answer 403 Editor permissions required. An archived board stays readable by its explicit members (archivedAt set) although GET /api/boards no longer lists it — archiving is listing, not privacy. Every board id the caller cannot read — private, archived, another workspace's, unknown — answers the one 403 No board with this id, or you are not a member of it, which never says whether the board exists.