Add CI checks for monorepo

[GitGab19]

Summary

• Adds a CI GitHub Actions workflow for the monorepo branch.
• Runs backend checks with Python 3.11: dependency install, Ruff formatting check, Ruff lint, and pytest.
• Runs frontend checks with Node 22: npm ci and npm run build.
• Adds a Docker image build check for the combined app image.

Stacked PR

This is stacked on top of monorepo-subtree, which is currently the head branch for edge-mining#34. Keeping this as a separate PR lets the monorepo port stay focused while reviewing CI independently.

Validation

• git diff --check monorepo-subtree...ci/pr-checks
• verified the stacked diff contains only .github/workflows/ci.yml

Not run locally: backend/frontend test suites, because local dependencies are not installed in this checkout.