Function atomic_enqueue 

pub async fn atomic_enqueue<'e, E>(
    executor: E,
    cron_name: &str,
    fire_time: DateTime<Utc>,
    previous_enqueued_at: Option<DateTime<Utc>>,
) -> Result<Option<JobRow>, AwaError>
where
    E: PgExecutor<'e>,

Atomically mark a cron job as enqueued AND insert the resulting job.

Uses a single CTE so that both the UPDATE and INSERT happen in one atomic operation. If the process crashes mid-transaction, Postgres rolls back both. If another leader already claimed this fire time (last_enqueued_at no longer matches), the UPDATE matches 0 rows and the INSERT produces nothing.

Returns the inserted job row, or None if the fire was already claimed.