What's the word/expression for a streak/sliver of light peeking through the space beneath the door?
A thin sliver of light is often referred to as a ‘crack of light’, but you would then have to specify where that crack was, it could be, for example ‘between the curtain’ or ‘around the shutters’. So you could describe this as ‘a crack of light under the door’ or ‘a crack of light at the threshold’.
An example of this usage is given in Collins online English dictionary:
She saw a crack of light under the bathroom door and opened it.