Sadly, this is a very complex issue not only for nostr  but for the entirety of the internet and offline. First off, thank you for sharing and bringing this up because I had the same questions and posted it on here. Second, I strongly believe that concerned users like us would play a massive part too to combat this type of content better than any AI or ML. I actively report contents like this and I came across it few hours ago. I always make an effort to report these things. Like you said, it is in the best interests of any hosting owner to make sure they do not host "illegal" content such as CSAM. Atm, I have no better answers or one solution that fits all the scenario. But I have faith that collectively we use our own discernment to report this kind of content. Ty! ❤