Function send 

pub async fn send(
    db: &Db,
    user_id: i64,
    message: &str,
    url: &str,
) -> Result<i64>

Persist one notification targeted at user_id. url may be empty when the message stands on its own (no click-through). Returns the new row’s id on success.

Project code calls this anywhere a Db is in scope — request handlers, background jobs, periodic tasks. The framework’s own audit pipeline does not emit notifications today; this is a project-facing surface.