Undo the latest review

Write. Undo the latest active review on a card this token owns. Body may be {} or { reviewId }. Unknown keys are rejected. Restores dueAt and the learning or review state to what they were before that grade. The review row stays, with undoneAt set, so export still lists it. A second call with the same reviewId returns alreadyUndone true and does not apply twice. A call with no reviewId undoes the new latest active review. When no active review remains, the response is 200 with nothingToUndo true. A reviewId that is not the latest active review is 409. Undo that later review first. A retired card is 404, the same as get and reveal. A card in an archived deck is 404 until that deck is unarchived. Another token's card is 404. The response includes the prompt and does not include the answer. There is no idempotency key. Retrying the original recordReview key after undo returns the original receipt and does not reapply the grade.