Verify yarn scripts run correctly and add Prettier/ESLint config

- [x] Run `yarn dev` and confirm service boots locally.
- [x] Run `yarn build` and confirm successful TypeScript build.
- [x] Run `yarn start` (from build) and confirm service starts.
- [x] Run `yarn test` and confirm tests pass and exit cleanly.
- [x] Fix any tsconfig/jest config issues if they appear.
- [x] Document common commands in README (“Development” section).
- [x] Add Prettier configuration file (`.prettierrc` or equivalent).
- [x] Add ESLint configuration file (`.eslintrc.json`) and extends from `@dcl/eslint-config/core-service`.
- [x] Ensure `yarn lint` and `yarn format` scripts exist and run correctly.
- [x] Validate that CI runs linting and formatting checks.