Increase unit test coverage

[sbidoul]

Low hanging fruits:

• [ ] db.py
• [ ] api.py
• [ ] webui.py
• [ ] slugify in utils.py

Somewhat harder:

• [ ] github.py (need to mock the http GitHub api endpoints, see what httpx has to offer, or use vcrpy)
• [ ] models.py (do we need #4 to test the models ?)
• [ ] k8s.py (depends on #4)