Function validate_user

pub async fn validate_user(
    user: &User,
    channel_id: &Uuid,
    app_state: &AppState,
) -> Result<RouterType, ApiError>

Perform some validations on the user, including:

• Validate router type
• (WebPush) Check that the subscription/channel exists
• (WebPush) Drop user if inactive

Returns an enum representing the user’s router type.