PERF: Move user-tips and narrative to per-user messagebus channels (#19773)

Using a shared channel with per-message permissions means that every client is updated with the channel's 'last_id', even if there are no messages available to them. Per-user channel names avoid this problem - the last_id will only be incremented when there is a message for the given user.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/new_user_narrative.rb

@@ -173,7 +173,11 @@ module DiscourseNarrativeBot
       topic = @post.topic
       post = topic.first_post
 
-      MessageBus.publish("/new_user_narrative/tutorial_search", {}, user_ids: [@user.id])
+      MessageBus.publish(
+        "/new_user_narrative/tutorial_search/#{@user.id}",
+        {},
+        user_ids: [@user.id],
+      )
 
       raw = <<~MD
       #{post.raw}