chore: add CI pipeline with lint, build and test

[lmoraesdev]

What

Adds a minimal CI pipeline that runs on every push to main and on every pull request targeting main:

1. npm ci — install with locked versions
2. npm run lint — ESLint + Prettier rules across src/ and test/
3. npm run build — NestJS build via SWC, followed by tsc-alias to rewrite @/ paths
4. npm test — full Vitest suite (unit + e2e)

Also adds ESLint and Prettier to the project, configured for TypeScript + NestJS.

Why

Before the next features land (structured logging, observability), the project needs a safety net that catches regressions automatically. A green CI badge on the README also signals to anyone reading the repo that the code on main actually compiles and passes its tests — not always a given in portfolio repos.

Approach

Three atomic commits:

1. chore: add lint script with eslint and prettier — ESLint flat config (eslint.config.mjs, the v9+ format), Prettier rules, lint/lint:fix/format scripts. Prettier reformatted 12 existing files (trailing commas, quotes); all tests still green afterward.
2. chore: add GitHub Actions CI workflow — .github/workflows/ci.yml running on Node 20 with npm cache enabled
3. docs: add CI status badge to README

Follow-up

After merging, enable branch protection on main requiring the CI check to pass before merge can happen. This makes the pipeline more than decorative — it actually gates the codebase.