fix(referral-service): 添加 WalletServiceClient 初始化日志

方便排查 WALLET_SERVICE_URL 环境变量是否正确加载

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

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

backend/services/referral-service/src/infrastructure/external/wallet-service.client.ts

@@ -21,6 +21,7 @@ export class WalletServiceClient {
     this.baseUrl =
       this.configService.get<string>('WALLET_SERVICE_URL') ||
       'http://localhost:3002';
+    this.logger.log(`WalletServiceClient initialized with baseUrl: ${this.baseUrl}`);
   }
 
   /**