FRE-5400: Consolidate webhook secret to single config source

WebhookService.constructEvent now reads from config.stripe.webhookSecret
instead of process.env.STRIPE_WEBHOOK_SECRET, matching BillingService.handleWebhook.

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

packages/shared-billing/src/services/billing.services.ts

@@ -174,7 +174,7 @@ export class WebhookService {
     return stripe.webhooks.constructEvent(
       rawBody,
       signature,
-      process.env.STRIPE_WEBHOOK_SECRET!
+      config.stripe.webhookSecret
     );
   }