create and get categories with database for user

backend/model/Category.go

@@ -4,7 +4,7 @@ type Category struct {
 	BaseModel
 	Icon          string        `json:"icon"`
 	Title         string        `json:"title"`
-	Subcategories []Subcategory `json:"subcategories" gorm:"foreignKey:CategoryID"`
+	Subcategories []Subcategory `json:"subcategories,omitempty" gorm:"foreignKey:CategoryID"`
 	Type          string        `json:"type"`
 	UserID        string
 }