Stored by createInvite, and read back by validateInvite when the invite
link is opened or used to sign up.
token: string
The random token the invite link carries, which is the primary key.
inviter_id: string
Becomes the new user's invited_by.
used_at: string | null
Set when an account is created with the invite.
created_at: string
Starts the 7-day window in which the invite works.