fix(backend): optional settings

[AntonioVentilii]

Motivation

In PR #3770 , i introduced the field settings in the UserProfile structure. I made it defaulting to avoid missing data in profiles created before the change.

However this caused issues with deserialization: the decoding was failing because data were missing obviously.

I had a few options:

1. migrate old data to the new struct --> not an option, not necessary, and a lot of effort/risk for something this small
2. deserialize UserProfile differently from the rest --> not worth it since the deserialization should not be affected by how a specific structure changes; furthermore, we DO want to raise errors if the deserialization fails, and not provide a certain default
3. use optionality --> For The Win

Changes

• Change the settings files to be optional
• Adapt consequent structures
• Remove Default trait

Tests

Current tests worked as intended.