Update README and CI/CD configuration for DigitalOcean deployment

Commit notes

- Changed backend deployment instructions in README to reflect the transition from Railway to DigitalOcean App Platform. - Added a new `.do/app.yaml` file for DigitalOcean App configuration, including environment variables and health checks. - Introduced a GitHub Actions workflow in `.github/workflows/ci.yml` for CI/CD, running tests and deploying to DigitalOcean on successful pushes to the main branch. - Updated environment example files to include `BACKEND_PUBLIC_URL` for local development and production configurations.