An account that signs up through an invitation works on the inviter's
billing, so handing it a trial at signup spent its only trial before it
owned anything. The trial is now held back for collaborators and claimed
only explicitly: a Start Free Trial button on the new-workspace and
billing screens calls the new POST /api/billing/trial endpoint, which
grants the once-per-account trial atomically. Nothing starts the clock
as a side effect, and pure collaborators no longer see a trial-ending
banner about work that is not theirs.
