Introduce service layer entities and extract business logic to services

[warrkan]

In Geti Tune backend, refactor the code to achieve a proper separation of the following three tiers and entities:

• DB layer: the classes are stored in app/db/, they define the database schema
• Service layer: the models are stored in app/models, they are used in services which implement the business logic
• Presentation layer: the models are stored in app/api/schemas, they define the schema of the REST API