docs: add status badges to README (npm, CI, license)

[PaulGrimshaw]

Context

The README has no badges. Badges give a quick health signal at a glance and are expected on any actively maintained OSS package.

Badges to add (top of README)

• npm version badge for @dcb-es/event-store
• npm version badge for @dcb-es/event-store-postgres
• CI status badge (once GitHub Actions workflow exists — see #[CI issue])
• License badge (MIT)

Example:

[![npm](https://img.shields.io/npm/v/@dcb-es/event-store)](https://www.npmjs.com/package/@dcb-es/event-store)
[![CI](https://github.com/sennentech/dcb-event-store/actions/workflows/ci.yml/badge.svg)](https://github.com/sennentech/dcb-event-store/actions/workflows/ci.yml)
[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](LICENSE.md)