The Nostr client would embed the torrent client.

This is a common practice with the protocol in some apps. e.g: Steam uses bittorrent for application/update downloads.

Up to the client to handle the seeding and the rules to apply ( service workers to keep seeding in the background for X time even if user is not on the app actively ).

The idea is that this way the files content wouldn't be in the nostr protocol and it wouldn't require additional protocol design. It'd be up to the client to handle the content retrieval, sharing, caching, etc.

Obviously there should be a way for user to fine-tune the behavior of the client regarding the torrent client, but i see this as another matter