Change markers API to use a replica (#25851)

app/controllers/api/v1/markers_controller.rb

@@ -7,7 +7,10 @@ class Api::V1::MarkersController < Api::BaseController
   before_action :require_user!
 
   def index
-    @markers = current_user.markers.where(timeline: Array(params[:timeline])).index_by(&:timeline)
+    with_read_replica do
+      @markers = current_user.markers.where(timeline: Array(params[:timeline])).index_by(&:timeline)
+    end
+
     render json: serialize_map(@markers)
   end