Add ruff linter and formatter to pre-commit and CI

[markuslf]

Add ruff (linter + formatter) to enforce consistent code style across the repository.

What to do

• Add ruff-check and ruff-format hooks to .pre-commit-config.yaml
• Add [tool.ruff] configuration to pyproject.toml with:
  • target-version = 'py39'
  • quote-style = 'single' (single quotes for strings, """ for triple-quoted)
  • Lint rules: E, W, F, I, B, C4, UP, SIM, RUF
• Apply ruff format to all Python files
• Fix any ruff check violations

Reference

See monitoring-plugins/pyproject.toml and firewallfabrik/pyproject.toml for the configuration pattern.