Expose SubscriptionState on License via JWT

[lucasfischer]

Adds a subscriptionState: SubscriptionState? field to License, populated from the signed JWT payload. nil means a perpetual / one-time-purchase license; otherwise it reflects the Stripe subscription lifecycle.

SubscriptionState is an enum with associated dates covering the full lifecycle:

• .renewing(renewsAt:): happy path
• .canceling(endsAt:canceledAt:): cancellation scheduled, access until endsAt
• .trialing(trialEndsAt:canceledAt:): in trial; converts to paid unless canceled
• .pastDue(gracePeriodEndsAt:): payment failed, Stripe retrying
• .paused
• .lapsed — terminal (canceled / unpaid / expired)

[simonlouschueler]

Looks great—thank you for implementing it. Especially .canceling(endsAt:canceledAt:) will be very useful to give the user a good overview of the current state of their subscription. We’re arriving at a state where it’s not only simpler to implement than StoreKit, but provides all the same features.