Introspect the current token

Returns who the token belongs to and what it may do:

• uid and email: the account the token was created in.
• plan: planId, state, trialEndsAt and currentPeriodEnd of the subscription.
• scopes: the scopes this token was granted.
• scheduling.timezones: the account time zone per connected platform, which is what ad-schedule hours are anchored to.

Call this first to confirm a token works and to learn its scopes before planning writes. Needs no scope and ignores X-Space-Id.