fix(referral-service): 修复 Kafka 消费异常被吞掉的问题

- kafka.service.ts: 抛出异常让 KafkaJS 触发重试
- user-registered.handler.ts: 传播异常到 KafkaService

修复前处理失败的消息不会重试，导致推荐关系可能丢失

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

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

backend/services/referral-service/src/application/event-handlers/user-registered.handler.ts

@@ -88,6 +88,7 @@ export class UserRegisteredHandler implements OnModuleInit {
         `Failed to create referral relationship for accountSequence=${payload.accountSequence}:`,
         error,
       );
+      throw error; // 传播异常，让 Kafka 重试
     }
   }
 }

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

@@ -100,6 +100,7 @@ export class KafkaService implements OnModuleInit, OnModuleDestroy {
           }
         } catch (error) {
           this.logger.error(`Error processing message from ${topic}:`, error);
+          throw error; // 让 KafkaJS 知道处理失败，触发重试
         }
       },
     });