Fix db relations, add sample data, show categories in frontend menu

software/backend/models/category.model.ts

@@ -0,0 +1,14 @@
+import { Table, Column, Model, BelongsTo, ForeignKey, HasMany } from 'sequelize-typescript';
+import { Product } from './product.model';
+
+@Table
+export class Category extends Model {
+  @Column
+  name: string
+
+  @Column
+  icon: string
+
+  @HasMany(() => Product)
+  product: Product
+}