tags の論理削除対応（#287） (#288)

#287

#287

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #288

backend/db/migrate/20260311123100_add_discarded_at_to_tags.rb

@@ -0,0 +1,6 @@
+class AddDiscardedAtToTags < ActiveRecord::Migration[8.0]
+  def change
+    add_column :tags, :discarded_at, :datetime
+    add_index :tags, :discarded_at
+  end
+end