feat: Implement JSON-backed database

src/db/user.rs

@@ -1,4 +1,4 @@
-use serde::Serialize;
+use serde::{Deserialize, Serialize};
 
 use std::fmt;
 
@@ -54,7 +54,7 @@ impl fmt::Display for UserRef {
     }
 }
 
-#[derive(Clone, Debug, Serialize)]
+#[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct User {
     /// Username, without the domain part. Once set, cannot be edited.
     pub username: String,