registerRecoveryKey

Registers publicKey (32 bytes) as userId's recovery key:

• the user never registered one: stores it at epoch 1, returns true;

• the user's key was revoked at epoch N: stores it at epoch N + 1 and clears the transition IDs, returns true; at N = Long.MAX_VALUE throws LastDeviceRecoveryKeyException.EpochExhausted;

• exactly this key is active: returns false, nothing changes;

• another key is active: throws LastDeviceRecoveryKeyException.Conflict, nothing changes. Only rotateRecoveryKey replaces an active key.

registeredAt (server time) becomes the key's installation time. A stored key also removes any pending reset of the user (none can exist without an active key). Concurrent calls behave as if they ran one after the other.