fix(billing): add err.stack + consistent stripeSessionId key to backfill logs

Fixes two logger.error calls (lines 332 and 344 of billing.webhook.service.js):
- rename sessionId key to stripeSessionId for consistency with the rest of the file
- add stack: err?.stack to both meta objects, matching surrounding logger.error style

Also updates billing.refund-correlation unit test header comment to reference
BillingFailedBackfillRepository.record instead of the stale BillingFailedBackfill.create.

Author: PierreBrisorgueil

modules/billing/services/billing.webhook.service.js

@@ -328,7 +328,7 @@ const handleCheckoutPaymentCompleted = async (session) => {
       } catch (err) {
         logger.error(
           '[billing.webhook] PI metadata backfill failed after retries — refund correlation at risk',
-          { paymentIntentId, sessionId: stripeSessionId, error: err?.message ?? String(err) },
+          { paymentIntentId, stripeSessionId, error: err?.message ?? String(err), stack: err?.stack },
         );
         try {
           await BillingFailedBackfillRepository.record({
@@ -340,7 +340,7 @@ const handleCheckoutPaymentCompleted = async (session) => {
         } catch (dlqErr) {
           logger.error(
             '[billing.webhook] dead-letter write failed — manual reconciliation required',
-            { paymentIntentId, sessionId: stripeSessionId, error: dlqErr?.message ?? String(dlqErr) },
+            { paymentIntentId, stripeSessionId, error: dlqErr?.message ?? String(dlqErr), stack: dlqErr?.stack },
           );
         }
       }

modules/billing/tests/billing.refund-correlation.unit.tests.js

@@ -9,7 +9,7 @@ import { retryWithBackoff } from '../lib/billing.retry.js';
  *
  * Covers:
  *   - retryWithBackoff: 3 attempts on Stripe API failure before escalating
- *   - Dead-letter: BillingFailedBackfill.create called after 3 consecutive failures
+ *   - Dead-letter: BillingFailedBackfillRepository.record called after 3 consecutive failures
  *   - Dead-letter write failure: logger.error emitted, handler does not throw
  */