fixed 评论回复增加parent_id索引，解决性能问题

db/migrate/20231115126452_add_journal_parent_id_index.rb

@@ -0,0 +1,5 @@
+class AddJournalParentIdIndex < ActiveRecord::Migration[5.2]
+  def change
+    add_index :journals, :parent_id
+  end
+end