fix(leaderboard-service): 修复 DI 注入问题 - 使用 useExisting 替代 useClass

- ReferralServiceClient 和 IdentityServiceClient 使用 useExisting
- 避免在 DomainModule 中重新创建实例导致 HttpService 不可用
- 复用 InfrastructureModule 中已创建的实例

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

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

backend/services/leaderboard-service/src/modules/domain.module.ts

@@ -11,11 +11,11 @@ import { IdentityServiceClient } from '../infrastructure/external/identity-servi
   providers: [
     {
       provide: REFERRAL_SERVICE_CLIENT,
-      useClass: ReferralServiceClient,
+      useExisting: ReferralServiceClient,
     },
     {
       provide: IDENTITY_SERVICE_CLIENT,
-      useClass: IdentityServiceClient,
+      useExisting: IdentityServiceClient,
     },
     LeaderboardCalculationService,
     VirtualRankingGeneratorService,