Don't use `@Entity`s directly for (de)serialization, use DTOs instead

Certain classes can be marked as opted-out of serialization (maybe @JsonIgnoreType?) and their usages in transport replaced with

• Execution -> ExecutionDto
• User -> UserInfo (consider #1130 as an example of why this is needed)

Another idea would be to create a custom detekt rule that would check that

• methods annotated with various @...Mapping return not-@Entity class or Mono
• bodyValue(), ObjectMapper.writeValue etc. are used with not-@Entity classes