Integrate Cap as a CAPTCHA provider

An option to use

https://github.com/tiagorangel1/cap as CAPTCHA would be excellent:

• Maximally privacy-preserving as it does not rely on off-site behavior tracking for risk assessment.

• Lightweight and easy to integrate.

• Self-hostable.

With it being so lightweight, it could potentially be integrated directly into OpnForm, alternatively OpnForm could rely on standalone-mode REST interface and then it would simply be a question of adding a Cap container to the Compose.