fix(champs): use approximate count in the task

app/tasks/maintenance/fill_champs_stable_id_task.rb

@@ -3,11 +3,16 @@
 module Maintenance
   class FillChampsStableIdTask < MaintenanceTasks::Task
     def collection
-      Champ.includes(:type_de_champ)
+      Champ.where(stable_id: nil).includes(:type_de_champ)
     end
 
     def process(champ)
       champ.update_columns(stable_id: champ.stable_id, stream: 'main')
     end
+
+    def count
+      sql = "SELECT reltuples FROM pg_class WHERE relname = 'champs';"
+      Champ.connection.select_value(sql).to_i
+    end
   end
 end