FEATURE: Publish read topic tracking events for private messages. (#14274)

Follow-up to fc1fd1b416

app/models/concerns/topic_tracking_state_publishable.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module TopicTrackingStatePublishable
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def publish_read_message(message_type:,
+                             channel_name:,
+                             topic_id:,
+                             user:,
+                             last_read_post_number:,
+                             notification_level: nil)
+
+      highest_post_number = DB.query_single(
+        "SELECT #{user.staff? ? "highest_staff_post_number" : "highest_post_number"} FROM topics WHERE id = ?",
+        topic_id
+      ).first
+
+      message = {
+        message_type: message_type,
+        topic_id: topic_id,
+        payload: {
+          last_read_post_number: last_read_post_number,
+          notification_level: notification_level,
+          highest_post_number: highest_post_number
+        }
+      }.as_json
+
+      MessageBus.publish(channel_name, message, user_ids: [user.id])
+    end
+  end
+end