Actually:
to.save(validate: false)
… did work just fine.
Edit: The GUI doesn’t show it immediately (I guess sidekiq caching). I did a ./launcher restart app
for that.
For completion:
to=TranslationOverride.where(translation_key:"user_notifications.user_watching_category_or_tag.text_body_template").first
to.value="%{username} posted in \"[%{topic_title}](%{base_url}%{url})\"\n\n--"
to.save(validate: false)