It is not clear to me what happens inside the stepper motor when it is manually and rapidly rotated in clockwise/counterclockwise, overcoming the torque of the motor.
As I understand, fly-back diodes are protecting the driver when the windings are not energized and the stepper is acting like a generator. So it seems to me that no additional protection is necessary if I manually rotate the shaft of the motor when it is fully powered. But I'm not 100% sure on this...
I plan to use an 8-diodes-array to protect the driver board: