Function enqueue 

pub async fn enqueue(
    conn: &DatabaseConnection,
    queue: &str,
    job_type: &str,
    payload: &str,
    max_retries: u32,
    idempotency_key: Option<&str>,
    tenant_id: Option<i64>,
    available_at: DateTime<Utc>,
) -> Result<(), Error>

Insert a new job into the queue.

When idempotency_key is Some, skips the insert if a pending or claimed row with the same (job_type, idempotency_key) already exists (D-15, T-185-01).