Add Docker Compose generation

[caydyan]

/claim #25

Summary

• Add a Docker Compose generator that derives a docker-compose.yml from the generated or cached devcontainer.json.
• Display the Docker Compose output alongside the devcontainer result with the existing copy-button behavior.
• Support image-based and Dockerfile/build-based devcontainer configs, forwarded ports, local environment references, and a safe base-image fallback for unparsable JSON.
• Keep the compose generation out of the database schema so cached and newly generated records both work without a migration.

Tests

python3 -m unittest discover -s tests
python3 -m compileall helpers main.py tests/test_docker_compose_generation.py
git diff --check

Repository note

Algora links this bounty to daytonaio/devcontainer-generator#25, but the live GitHub API returns 404/disabled issues for that repository path. This PR targets the public repository currently exposing the codebase, nibzard/devcontainer-generator, matching the existing claim PR pattern in this repo.