[CI-01] Configure GitHub Actions quality checks

[Krisor1986]

Goal

Automatically validate every pull request before merge.

Tasks

• Run backend tests
• Run frontend build and tests
• Add lint and formatting checks
• Cache dependencies
• Document required checks
• Protect secrets from logs

Acceptance criteria

• Checks run on pull requests to develop
• A failing test or build blocks completion
• Results are visible from the pull request