When processing checkout.session.completed events for subscription-based checkouts:

1. The webhook JSON had payment_intent: null since subscription checkouts use a subscription ID instead of a payment intent ID
2. The calculateCommission method was failing to properly handle missing commission data in the partner program array
3. The controller wasn't looking for the commission fields (commission_type and commission_value) in the SQL query
4. The method was returning null instead of a float as defined in the return type