Add CI workflow and protect main branch workflow

[FriggemannMichael]

Objective:
Set up the repository workflow so all feature work happens on feature branches and tests run before merging into main.

Acceptance criteria:

• A GitHub Actions workflow runs on pull requests targeting main.
• The workflow installs dependencies from requirements.txt.
• The workflow runs Django system checks.
• The workflow runs Ruff linting.
• The workflow runs Ruff format checks.
• The workflow runs pytest.
• main is protected after the workflow exists.
• Direct pushes to main are avoided; work happens through feature branches and pull requests.

TDD workflow:

• CI must support the TDD cycle by running the test suite on every PR.
• Tests should be added before implementation work in feature branches.