Use pydantic's `from_orm` to fill in schema values, instead of the `schema` property on the model classes

[Crsi]

This goal may be hard or almost impossible to achieve, since the schemas heavily differ from the database models:

• the database models have different attribute names than the schemas (e.g. aliases)
• the schemas provide more attributes than the native database models, which are generated upon instantiation (e.g. refunds)