Function placeholders 

async fn placeholders(
    base_url: &str,
    conn: &mut PgConnection,
    notification: &StudentNotificationToQueue,
    product_urls: &mut ProductUrlCache,
) -> Result<Value>

Stored on the delivery row, so the sender needs no lookup of its own.

ENROLMENT_LINK is empty when the module has no product or no resolved token; the template’s sentence has to read correctly without it, because a mail that only says “enrol in Sisu” is all the student gets in that case.