OUCH!
I did the same thing tensioning the chain on my motorbike ("Fizzie" moped to be more accurate). It was 30 years ago but it was the sort of pain that makes me wince even thinking about it today.
I can't give you any advice, just sympathy. I had to make the decision whether to rotate the back wheel and get my finger out or whether to just live with the (slightly less) pain and have a moped attached to my finger for the rest of my life.
