Make License.expiresAt the license end date

[lucasfischer]

Summary

License.expiresAt now gives the license end date, not the token expiry. The value comes from a new expires_at claim in the token.

Follow-up changes:

• Grace does not apply to a license past its end date, unless the subscription renews.
• The checkout receipt shows "Trial ends" for a trial that converts, and "Lifetime" for a purchase with no end date.

Breaking change

License.expiresAt is nil for a license that never expires and for a token minted before the claim existed. Code that force-unwraps it can crash.