Improve api/v1/markers#create performance against simultaneous requests (#28718)

app/controllers/api/v1/markers_controller.rb

@@ -19,7 +19,7 @@ class Api::V1::MarkersController < Api::BaseController
       @markers = {}
 
       resource_params.each_pair do |timeline, timeline_params|
-        @markers[timeline] = current_user.markers.find_or_initialize_by(timeline: timeline)
+        @markers[timeline] = current_user.markers.find_or_create_by(timeline: timeline)
         @markers[timeline].update!(timeline_params)
       end
     end