fix(contribution): run CDC sync in background to avoid blocking service startup

- Change await to .then() for cdcConsumer.start()
- Allows HTTP endpoints to be accessible during CDC sync

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

backend/services/contribution-service/src/application/event-handlers/cdc-event-dispatcher.ts

@@ -51,14 +51,14 @@ export class CDCEventDispatcher implements OnModuleInit {
       this.handleAdoptionPostCommit.bind(this),
     );
 
-    // 启动 CDC 消费者
-    try {
-      await this.cdcConsumer.start();
+    // 启动 CDC 消费者（非阻塞，在后台运行顺序同步）
+    // 不能 await，否则会阻塞服务启动，导致 HTTP 端点无法访问
+    this.cdcConsumer.start().then(() => {
       this.logger.log('CDC event dispatcher started with transactional idempotency');
-    } catch (error) {
+    }).catch((error) => {
       this.logger.error('Failed to start CDC event dispatcher', error);
       // 不抛出错误，允许服务在没有 Kafka 的情况下启动（用于本地开发）
-    }
+    });
   }
 
   private async handleUserEvent(event: CDCEvent, tx: TransactionClient): Promise<void> {