DEV: Add API scope for categories endpoint (#16105)

This change adds support for the categories endpoint to have an api
scope. Only adds GET scope for listing categories and for fetching a
single category.

See: https://meta.discourse.org/t/218080/4

app/models/api_key_scope.rb

@@ -36,6 +36,10 @@ class ApiKeyScope < ActiveRecord::Base
         posts: {
           edit: { actions: %w[posts#update], params: %i[id] }
         },
+        categories: {
+          list: { actions: %w[categories#index] },
+          show: { actions: %w[categories#show], params: %i[id] }
+        },
         uploads: {
           create: {
             actions: %w[