fix(docker): 修复 referral-service Dockerfile 健康检查 URL

修复 referral-service 的健康检查端点配置：
- referral-service: /health -> /api/v1/health

注：backup-service 使用 /health，leaderboard-service 使用 /api/health（这是服务本身实现的端点）

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

backend/services/referral-service/Dockerfile

@@ -75,7 +75,7 @@ EXPOSE 3004
 
 # Health check
 HEALTHCHECK --interval=30s --timeout=3s --start-period=60s --retries=3 \
-  CMD curl -f http://localhost:3004/health || exit 1
+  CMD curl -f http://localhost:3004/api/v1/health || exit 1
 
 # Start service with migration
 CMD ["/app/start.sh"]