This should be really easy to do! But I am very busy with IRL things these days. So, here's some free thoughts:

- Flat database (SurrealDB, MongoDB)

- endpoints /api/register to finalize a registration prepped in the frontend; perhaps by first proving Bitcoin ownership or something? I dont know how "walled" you want it to be in that regard.

- Handle /.well-known/nostr.json?username=$name by just looking it up in the database.

- A registered user may interact with /api/update to change relay info.

- Authentication can be done using the nostr window object - in fact, this way you can have the user sign something using their nsec and validate against their npub to check authenticity.

Good luck!!