FRE-5401: Migrate webhook idempotency to distributed Redis store

Replace in-memory Map<string, number> with Redis-based idempotency
using setIfNotExists (NX) for distributed multi-instance deployments.
Removes cleanupOldEvents (no longer needed with Redis TTL).

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

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

@@ -1,21 +1,12 @@
 import Stripe from 'stripe';
 import { loadBillingConfig, SubscriptionTier } from '../config/billing.config';
+import { RedisService } from '@shieldsai/shared-notifications';
 import type { Subscription, SubscriptionCreateSchema, SubscriptionUpdateSchema } from '../models/subscription.model';
 
 const config = loadBillingConfig();
 const stripe = new Stripe(config.stripe.apiKey, { apiVersion: '2023-10-16' });
-
-const processedEvents = new Map<string, number>();
-const IDEMPOTENCY_TTL_MS = 24 * 60 * 60 * 1000;
-
-function cleanupOldEvents(): void {
-  const now = Date.now();
-  for (const [eventId, timestamp] of processedEvents.entries()) {
-    if (now - timestamp > IDEMPOTENCY_TTL_MS) {
-      processedEvents.delete(eventId);
-    }
-  }
-}
+const redis = RedisService.getInstance();
+const IDEMPOTENCY_TTL_SECONDS = 24 * 60 * 60;
 
 export class BillingService {
   private static instance: BillingService;
@@ -266,15 +257,17 @@ export class BillingService {
     body: Buffer
   ): Promise<Stripe.Event | null> {
     const event = stripe.webhooks.constructEvent(body, sig, config.stripe.webhookSecret);
-    
-    cleanupOldEvents();
 
-    if (processedEvents.has(event.id)) {
+    const wasNew = await redis.setIfNotExists(
+      `stripe:event:${event.id}`,
+      '1',
+      IDEMPOTENCY_TTL_SECONDS
+    );
+
+    if (!wasNew) {
       return null;
     }
 
-    processedEvents.set(event.id, Date.now());
-
     return event;
   }