feat: add postbuild script for auto-publish to local Verdaccio

When Verdaccio is running at localhost:4873, the package is automatically
published after each build. If Verdaccio isn't running, the build
completes normally without error.

Usage:
1. Start Verdaccio: docker compose up -d verdaccio (in ratings-stack)
2. First time only: npm adduser --registry http://localhost:4873
3. Build: npm run build (auto-publishes to Verdaccio)

Closes #4

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

)

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