If all you do is have the Arduino turn the key in a deadbolt, then I would wire that into a hobby servo of some sort. It's easy to make hobby servos turn 180 degrees from an Arduino, and they need no shield (as long as you power them separately correctly.)
A HiTec HS-311 or 322 might do fine, depending on how hard your deadbolt key is to turn. This is also probably safe enough, because the key remains there and can be turned by hand if needed.