Refactor models (added entity parent class)

api/src/engine/models/category.model.ts

-import { Field, ObjectType } from '@nestjs/graphql';
+import { ObjectType } from '@nestjs/graphql';
+import { Entity } from './entity.model';
 
 @ObjectType()
-export class Category {
-  @Field()
-  id: string;
-
-  @Field()
-  label: string;
-}
+export class Category extends Entity {}

api/src/engine/models/entity.model.ts

+import { Field, ObjectType } from '@nestjs/graphql';
+
+@ObjectType()
+export class Entity {
+  @Field()
+  id: string;
+
+  @Field({ nullable: true })
+  label: string;
+}

api/src/engine/models/group.model.ts

 import { Field, ObjectType } from '@nestjs/graphql';
+import { Entity } from './entity.model';
 import { Variable } from './variable.model';
 
 @ObjectType()
-export class Group {
-  @Field()
-  id: string;
-
-  @Field()
-  label: string;
-
+export class Group extends Entity {
   @Field({ nullable: true })
   description?: string;
 

api/src/engine/models/variable.model.ts

 import { Field, ObjectType } from '@nestjs/graphql';
 import { Category } from './category.model';
+import { Entity } from './entity.model';
 import { Group } from './group.model';
 
 @ObjectType()
-export class Variable {
-  @Field()
-  id: string;
-
-  @Field({ nullable: true })
-  label?: string;
-
+export class Variable extends Entity {
   @Field()
   type: string;