chore: add private:true to prevent npm publish (#4384)

[gibaros]

Summary

• Adds "private": true to package.json to prevent this service from being published to npmjs.com
• This service should only publish Docker images to hub.docker.com/u/mojaloop
• The CircleCI orb's check_npm_private gate will now skip the publish_npm job

Context

The mojaloop/build CircleCI orb always includes both publish_npm and publish_docker jobs. The publish_npm job gates itself by checking if package.json has "private": true. Without this flag, services with Dockerfiles were incorrectly publishing to both npm and Docker Hub.

Refs: mojaloop/project#4384

Test plan

• Verify jq '.private' package.json returns true
• On next release tag, confirm CircleCI publish_npm job logs "Package is private, skipping job"
• Confirm Docker publish still works normally

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