Containerize app and add CI workflow

Companion - Containerize and add CI

Apply DevOps practices to the Spring Boot + Vue monorepo.

Tasks

• Propose a plan for containerizing the app (single image vs. separate frontend/backend images) before implementing anything.
• Once the approach is agreed, write the Dockerfile(s) and confirm the app can be built and run in Docker.
• Add a GitHub Actions workflow that runs on every push: installs dependencies and runs tests for both backend and frontend.
• Make sure Docker build and CI steps correctly reference each part of the monorepo (frontend/backend paths).

Acceptance criteria

• A short plan/proposal exists and is reviewed before implementation starts.
• The app can be built and run via Docker, serving both the UI and the API.
• No secrets/credentials hardcoded — config via env vars.
• CI fails if either the backend or frontend test suite fails.
• CI run passes on a clean checkout with no dependency on local machine state.

Start by proposing and getting agreement on single versus separate images for the frontend/backend monorepo, then define the Dockerfile(s) and GitHub Actions workflow around the stated frontend/backend paths. Done means Docker builds and serves the UI and API, CI runs both test suites on every push, and the clean-checkout workflow uses environment variables rather than hardcoded secrets.