fix(docker-compose): 添加 referral-service 的 WALLET_SERVICE_URL 配置

referral-service 需要调用 wallet-service 确认扣款，
但缺少环境变量配置导致使用默认 localhost:3002 无法访问

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

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

backend/services/docker-compose.yml

@@ -319,6 +319,7 @@ services:
       - KAFKA_CLIENT_ID=referral-service
       - KAFKA_GROUP_ID=referral-service-group
       - AUTHORIZATION_SERVICE_URL=http://rwa-authorization-service:3009
+      - WALLET_SERVICE_URL=http://rwa-wallet-service:3001
     depends_on:
       postgres:
         condition: service_healthy