Function create_user 

pub async fn create_user(
    users_db: &Database,
    instance: &Instance,
    username: impl AsRef<str>,
    password: Option<&str>,
) -> Result<(String, UserInfo)>

Create a new user account

This function:

1. Optionally hashes the user’s password (if provided)
2. Generates a device keypair for the user
3. Creates a user database for storing keys (encrypted or unencrypted)
4. Creates UserInfo and stores it in _users database with auto-generated UUID

Arguments

• users_db - The _users system database
• instance - The Instance handle
• username - Unique username for login
• password - Optional password. If None, creates passwordless user (instant login, no encryption)

Returns

A tuple of (user_uuid, UserInfo) where user_uuid is the generated primary key