Offline Caching

[xeruf]

Describe the enhancement
In my nostr application, I would like to be able to retrieve previously seen events and queue created events offline on a per-relay basis, so they can be sent out and refreshed upon getting a connection again.
I thought it might be sensible to implement a mechanism like that in the SDK (simply serialize the database?) or maybe an integration with https://github.com/PrimalHQ/primal-server instead of implementing it directly in my application.
Of course negentropy is also an intriguing consideration here but maybe a bit out of scope.

Use case
Viewing and replying to nostr posts offline in any application - especially interesting with nostr utility applications like my task manager mostr