Excellent data, we are going to just bite the bullet and migrate this in core discourse to a bigint. It is an expensive move , but this will certainly pop up again on big forums so we might as well fix it.
Note … the OP is about post_id … overflowing that going to be a lot harder than notifications. 2.1 billion posts certainly going to happen, but the cost of biginting post_id is far more expansive than notification id. We can wait a bit on that time bomb.