fix(gro-609): fix two bugs found by CTO review

1. Refund stats now sum actual refund amounts from refunds table
   instead of incorrectly summing tip_cents from invoices table.

2. Stripe payment_intents.retrieve now expands payment_method
   so card.last4 is correctly available instead of null.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/services/payment.ts

@@ -169,7 +169,7 @@ export async function getPaymentIntentDetails(
   const stripe = getStripeClient();
   if (!stripe) return null;
 
-  const pi = await stripe.paymentIntents.retrieve(paymentIntentId);
+  const pi = await stripe.paymentIntents.retrieve(paymentIntentId, { expand: ["payment_method"] });
   const cardLast4 = pi.payment_method
     ? (pi.payment_method as Stripe.PaymentMethod).card?.last4 ?? null
     : null;