FRE-4521 Implement Redis integration for rate limiting and deduplication

- Add ioredis dependency for Redis connection pooling
- Create RedisService singleton with connection management
- Add Redis config (url, dedupWindowSeconds) to notification.config.ts
- Implement NotificationService.checkRateLimit using Redis INCR+EXPIRE
- Implement NotificationService.deduplicateNotification using Redis SET/NX
- Add configurable rate limit windows and thresholds via env vars
- Add 29 unit tests covering Redis operations, rate limiting, and dedup
- All tests pass, TypeScript compiles cleanly for new files

packages/shared-notifications/package.json

@@ -14,7 +14,8 @@
     "firebase-admin": "^12.0.0",
     "twilio": "^4.0.0",
     "zod": "^3.22.0",
-    "express": "^4.18.0"
+    "express": "^4.18.0",
+    "ioredis": "^5.4.0"
   },
   "devDependencies": {
     "@types/express": "^4.17.0",