샘플 토픽명으로 변경 (sample. 접두사 추가)

- 다른 서비스 개발자들의 운영 토픽과 충돌 방지
- MVP용 샘플 토픽: sample.event.created, sample.participant.registered, sample.distribution.completed
- KafkaTopicConfig, SampleDataLoader, 3개 Consumer 모두 업데이트

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

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

analytics-service/src/main/resources/application.yml

@@ -51,6 +51,11 @@ spring:
       enable-auto-commit: true
       key-deserializer: org.apache.kafka.common.serialization.StringDeserializer
       value-deserializer: org.apache.kafka.common.serialization.StringDeserializer
+    producer:
+      key-serializer: org.apache.kafka.common.serialization.StringSerializer
+      value-serializer: org.apache.kafka.common.serialization.StringSerializer
+      acks: all
+      retries: 3
     properties:
       connections.max.idle.ms: 10000
       request.timeout.ms: 5000