fix(contribution-service): 修复Kafka消息BigInt序列化错误

JSON.stringify无法序列化BigInt，添加自定义replacer将BigInt转换为字符串

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

backend/services/contribution-service/src/infrastructure/kafka/kafka-producer.service.ts

@@ -25,7 +25,7 @@ export class KafkaProducerService implements OnModuleInit {
       await lastValueFrom(
         this.kafkaClient.emit(topic, {
           key: message.key,
-          value: JSON.stringify(message.value),
+          value: JSON.stringify(message.value, this.bigIntReplacer),
           headers: message.headers,
         }),
       );
@@ -36,6 +36,16 @@ export class KafkaProducerService implements OnModuleInit {
     }
   }
 
+  /**
+   * JSON.stringify replacer 函数，将 BigInt 转换为字符串
+   */
+  private bigIntReplacer(_key: string, value: any): any {
+    if (typeof value === 'bigint') {
+      return value.toString();
+    }
+    return value;
+  }
+
   async emitBatch(topic: string, messages: KafkaMessage[]): Promise<void> {
     try {
       for (const message of messages) {