The id of the attendee whose booking owns this ledger event group, or null when none does. The single-batch booking write stamps every one of an attendee's listing_attendees rows with the booking's ledger_event_group (in the same batch that posts the legs), so a paid session's event group resolves back to exactly the attendee it created. This lets an idempotent replay recover the existing booking from the durable ledger after the (prunable) processed_payments idempotency row has gone — without it, a replay whose legs already exist would be mistaken for a capacity failure and refund a live ticket.