Fix Stripe.net v51 breaking API changes

[Copilot]

Stripe.net 51.0.0 (API version 2026-03-25.dahlia) removed three properties used in StripeService, causing build failures after the version bump in #78.

Breaking changes addressed

• Subscription.CurrentPeriodEnd removed — moved to SubscriptionItem. ResolveCurrentPeriodEnd now reads from the first item, retaining the raw JSON fallback:

  var firstItem = sub.Items?.Data?.FirstOrDefault();
  var itemPeriodEnd = new DateTimeOffset(firstItem.CurrentPeriodEnd, TimeSpan.Zero);

• Invoice.SubscriptionId removed — restructured under Invoice.Parent. Updated MapInvoicePaymentFailedEvent:

  invoice.Parent?.SubscriptionDetails?.SubscriptionId

• Invoice.ChargeId removed — charges now live in the Invoice.Payments collection. CreateRefundAsync expands data.payments and selects the paid charge:

  Expand = ["data.payments"],
  // ...
  latestInvoice.Payments?.Data
      ?.FirstOrDefault(p => p.Status == "paid" && p.Payment?.ChargeId is not null)
      ?.Payment?.ChargeId

  Filtering by Status == "paid" avoids selecting a charge from a prior failed payment attempt.