fix(referral): add EventAckPublisher to InfrastructureModule

- Refactor EventAckPublisher to use KafkaService instead of ClientKafka
- Add kafka/index.ts export file
- Register EventAckPublisher in InfrastructureModule providers and exports

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/referral-service/src/infrastructure/index.ts

@@ -2,3 +2,4 @@ export * from './database';
 export * from './repositories';
 export * from './messaging';
 export * from './cache';
+export * from './kafka';

backend/services/referral-service/src/infrastructure/kafka/index.ts

@@ -0,0 +1 @@
+export * from './event-ack.publisher';

backend/services/referral-service/src/modules/infrastructure.module.ts

@@ -7,6 +7,7 @@ import {
   KafkaService,
   EventPublisherService,
   RedisService,
+  EventAckPublisher,
 } from '../infrastructure';
 import {
   REFERRAL_RELATIONSHIP_REPOSITORY,
@@ -21,6 +22,7 @@ import {
     KafkaService,
     RedisService,
     EventPublisherService,
+    EventAckPublisher,
     {
       provide: REFERRAL_RELATIONSHIP_REPOSITORY,
       useClass: ReferralRelationshipRepository,
@@ -35,6 +37,7 @@ import {
     KafkaService,
     RedisService,
     EventPublisherService,
+    EventAckPublisher,
     REFERRAL_RELATIONSHIP_REPOSITORY,
     TEAM_STATISTICS_REPOSITORY,
   ],