Fix /activity endpoint not require signature in authorized fetch mode (#15592)

Fixes #15589

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/controllers/statuses_controller.rb

@@ -8,7 +8,7 @@ class StatusesController < ApplicationController
 
   layout 'public'
 
-  before_action :require_signature!, only: :show, if: -> { request.format == :json && authorized_fetch_mode? }
+  before_action :require_signature!, only: [:show, :activity], if: -> { request.format == :json && authorized_fetch_mode? }
   before_action :set_status
   before_action :set_instance_presenter
   before_action :set_link_headers