Add ActivationClaim and ActivationCheckResult types plus five methods on
Account (RegisterActivation, CheckActivationExpiry, IsActivationExpired,
GetExpiredActivations, RemoveExpiredActivations) and an ActiveActivationCount
property, mirroring Go accounts.go checkActivation / activationExpired logic.
Adds 10 targeted tests in Auth/ActivationExpirationTests.cs (all pass).
