Function login_user 

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

Login a user

This function:

1. Searches for user by username in _users database
2. Verifies password (if provided and required)
3. Opens user’s private database
4. Loads and decrypts user keys (or loads unencrypted for passwordless users)
5. Creates UserKeyManager with keys
6. Returns User session object

Arguments

• users_db - The _users system database
• instance - The Instance handle
• username - Username for login
• password - Optional password. None for passwordless users.

Returns

A User session object with keys loaded