Function is_hard_send_failure 

pub fn is_hard_send_failure(
    retryable: bool,
    first_failed_at: Option<DateTime<Utc>>,
    now: DateTime<Utc>,
) -> bool

True once a delivery attempt has failed for good: not retryable at all, or retryable but its retry window has expired. Shared with credit_registration_account_linking_emails.rs so the two cannot drift on what counts as a hard failure.