Add deployment scripts and configuration for Digital Ocean App Platform

Commit notes

- Introduced a new `deploy-to-do.sh` script to streamline deployment to Digital Ocean, including pre-deployment checks and build testing. - Created `DIGITAL_OCEAN_DEPLOYMENT.md` as a comprehensive guide for deploying the Samplestep API, detailing prerequisites, setup, and troubleshooting. - Updated `.do/app.yaml` for Digital Ocean configuration, including environment variables and health checks. - Added `backend/build.sh` for building the Go application, ensuring a consistent build process. - Included `backend/.buildpacks` for specifying the Go buildpack. - Enhanced `.gitignore` to exclude Go build artifacts and environment files. - Created `backend/env.production.do` for production environment variable management. - Updated `backend/Procfile` to define the web process for deployment. - Modified `backend/go.mod` to include necessary dependencies for OAuth2. - Improved logging in `backend/cmd/api/main.go` to differentiate between development and production database connections.