feat: Category API to handle linked categories and parent-sub categories (#4879)

spec/models/related_category_spec.rb

@@ -0,0 +1,8 @@
+require 'rails_helper'
+
+RSpec.describe RelatedCategory, type: :model do
+  describe 'associations' do
+    it { is_expected.to belong_to(:category) }
+    it { is_expected.to belong_to(:related_category) }
+  end
+end