fix: Improve performance of most hit APIs in widget (#11089)

- Cache campaigns for better performance
- Fix N+1 queries in inbox members
- Remove unused related articles

app/controllers/api/v1/widget/inbox_members_controller.rb

@@ -2,6 +2,6 @@ class Api::V1::Widget::InboxMembersController < Api::V1::Widget::BaseController
   skip_before_action :set_contact
 
   def index
-    @inbox_members = @web_widget.inbox.inbox_members.includes(:user)
+    @inbox_members = @web_widget.inbox.inbox_members.includes(user: { avatar_attachment: :blob })
   end
 end