fix(billing): pin the Stripe API version and stop unpaid periods granting access

The Stripe client was built without an apiVersion, so the SDK followed whatever
version it shipped with. Two fields moved in the Basil API version: the billing
period went from the subscription onto its items, and the invoice link to its
subscription went under parent.subscription_details. Both reads returned
undefined without failing, which left stripeCurrentPeriodEnd null for every
subscriber and left the app with no invoice handling at all. A customer whose
card failed saw nothing about the invoice that was still retrying, and a
cancellation did nothing to stop those retries.

- Pin the API version, with `satisfies` so an SDK bump is a compile error here
  before it is a null read in production.
- Read the period off subscription items and the subscription off invoice
  parents, keeping the legacy fields as a fallback for older payloads.
- Handle invoice.paid, invoice.payment_failed, invoice.voided and
  invoice.marked_uncollectible through the existing customer-wide resync, so
  the mirror reflects payment health during dunning rather than after it.
- Add an in-app cancellation route: at period end when the subscription is
  paid, immediately plus voiding the open invoices when it is not, because
  cancelling alone does not stop collection on an invoice already issued.
- Ask Stripe, not just the local mirror, before opening checkout.
- Show the open invoice, the retry date and a payment-method-update shortcut in
  settings, and put a confirmation in front of cancellation.

Access no longer rests on the reported period alone. Stripe advances the period
when it issues the renewal invoice, paid or not, and the period survives
cancellation, so once the period field started being read correctly that check
would have handed a full free month to anyone whose renewal failed, and the new
cancel route would have let them void the invoice and keep the month. Access now
follows the subscription status, billingAccessEndedAt is enforced as a hard
cutoff in both hasBillingAccess and the query that mirrors it, and a subscription
behind on payment keeps access for Stripe's retry window rather than for the
period it never paid for.

app/api/billing/route.ts

@@ -1,6 +1,7 @@
+import { BillingSubscriptionStatus } from '@prisma/client';
 import { auth } from '@/lib/auth';
 import { apiErrors, successResponse, withCacheControl } from '@/lib/api-response';
-import { getBillingOverview } from '@/lib/billing';
+import { getBillingOverview, getOpenInvoiceForCustomer } from '@/lib/billing';
 import { isStripeFeatureEnabled } from '@/lib/feature-flags';
 import { hasStripeRuntimeConfig, isStripeConfigured } from '@/lib/stripe';
 import { logError } from '@/lib/logger';
@@ -15,12 +16,51 @@ export async function GET() {
     const billing = await getBillingOverview(session.user.id);
     const isEnabled = isStripeFeatureEnabled();
     const isConfigured = hasStripeRuntimeConfig();
+
+    // Only looked up when the account actually owes something, so the common path does not
+    // pay for a Stripe round trip.
+    const needsPaymentFix =
+      billing.subscription.status === BillingSubscriptionStatus.PAST_DUE ||
+      billing.subscription.status === BillingSubscriptionStatus.UNPAID;
+    const openInvoice =
+      isStripeConfigured() && needsPaymentFix && billing.subscription.stripeCustomerId
+        ? await getOpenInvoiceForCustomer(
+            billing.subscription.stripeCustomerId,
+            billing.subscription.stripeSubscriptionId
+          )
+        : null;
+
     const response = successResponse({
       isEnabled,
       isConfigured,
       status: !isEnabled ? 'disabled' : isStripeConfigured() ? 'ready' : 'misconfigured',
       checkoutAvailable: isStripeConfigured() && !billing.subscription.hasRecoverableSubscription,
-      portalAvailable: isStripeConfigured() && Boolean(billing.subscription.stripeCustomerId),
+      // A customer id alone is not enough: it is created on the first checkout attempt, so
+      // someone who abandoned checkout would be sent to an empty portal.
+      portalAvailable:
+        isStripeConfigured() &&
+        Boolean(billing.subscription.stripeCustomerId) &&
+        (billing.subscription.hasRecoverableSubscription ||
+          Boolean(billing.subscription.stripeSubscriptionId)),
+      // Gated on the status rather than on the mirrored subscription id: the id survives a
+      // cancellation until the deletion webhook arrives, and offering Cancel on an already
+      // canceled subscription just returns an error.
+      cancelAvailable:
+        isStripeConfigured() &&
+        billing.subscription.hasRecoverableSubscription &&
+        !billing.subscription.cancelAt &&
+        !billing.subscription.cancelAtPeriodEnd,
+      needsPaymentFix,
+      openInvoice: openInvoice
+        ? {
+            id: openInvoice.id,
+            hostedInvoiceUrl: openInvoice.hostedInvoiceUrl,
+            amountDue: openInvoice.amountDue,
+            currency: openInvoice.currency,
+            attemptCount: openInvoice.attemptCount,
+            nextPaymentAttempt: openInvoice.nextPaymentAttempt?.toISOString() ?? null,
+          }
+        : null,
       subscription: {
         status: billing.subscription.status,
         label: billing.subscription.label,