Sets or removes the project's default flag lifetime, in days (1–3650; null removes it).

Temporary flags with no expiry date of their own then expire that many days after they were created, reported as expirySource: "project-default". Takes effect at the next hourly staleness run. Requires at least Member, and the default-flag-lifetime feature to set one (400 FEATURE_NOT_ENABLED); removing one is always allowed. days is required; send {"days": null} to remove.