FRE-5348: Fix P1 billing issues

- Add null check for subscription items in updateSubscription
- Implement webhook handlers with Prisma DB persistence
- cancelSubscription already correctly passes cancel_at_period_end

All P1 issues validated and fixed. Ready for Security Review.

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

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

@@ -152,11 +152,19 @@ export class BillingService {
 
     const newTierConfig = config.tiers[newTier];
     
+    // Ensure subscription has items before updating
+    const firstItem = subscription.items.data[0];
+    if (!firstItem?.id) {
+      throw new Error(
+        `Subscription ${subscriptionId} has no items to update`
+      );
+    }
+
     const updated = await stripe.subscriptions.update(subscriptionId, {
       proration_behavior: 'create_prorations',
       items: [
         {
-          id: subscription.items.data[0]?.id,
+          id: firstItem.id,
           price: newTierConfig.priceId,
         },
       ],