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/src/index.ts

@@ -1,7 +1,8 @@
 export { EmailService } from './services/email.service';
 export { SMSService } from './services/sms.service';
 export { PushService } from './services/push.service';
-export { NotificationService } from './services/notification.service';
+export { NotificationService, RateLimitResult } from './services/notification.service';
+export { RedisService } from './services/redis.service';
 export { TemplateService } from './services/template.service';
 export { loadNotificationConfig, NotificationConfigSchema } from './config/notification.config';
 export { notificationRoutes } from './routes/notification.routes';