fix(contribution): run CDC sync in background to allow API access during sync

Change CDC consumer startup from blocking await to non-blocking .then()
so HTTP server starts immediately and /health/cdc-sync API is accessible
for deploy script to poll sync status.

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

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

@@ -51,14 +51,17 @@ export class CDCEventDispatcher implements OnModuleInit {
       this.handleAdoptionPostCommit.bind(this),
     );
 
-    // 启动 CDC 消费者
-    try {
-      await this.cdcConsumer.start();
-      this.logger.log('CDC event dispatcher started with transactional idempotency');
-    } 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）
-    }
+    // 非阻塞启动 CDC 消费者
+    // 让 HTTP 服务器先启动，CDC 同步在后台进行
+    // 脚本通过 /health/cdc-sync API 轮询同步状态
+    this.cdcConsumer.start()
+      .then(() => {
+        this.logger.log('CDC event dispatcher started with transactional idempotency');
+      })
+      .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> {