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/cancel/route.ts

@@ -0,0 +1,78 @@
+import { NextRequest } from 'next/server';
+import { auth } from '@/lib/auth';
+import { apiErrors, successResponse, withCacheControl } from '@/lib/api-response';
+import {
+  findLiveStripeSubscription,
+  getBillingOverview,
+  isUnpaidStripeSubscription,
+  syncStripeSubscriptionToUser,
+  voidOpenSubscriptionInvoices,
+} from '@/lib/billing';
+import { rateLimit } from '@/lib/rate-limit';
+import { isStripeFeatureEnabled } from '@/lib/feature-flags';
+import { getStripe, isStripeConfigured } from '@/lib/stripe';
+import { isTrustedSameOriginRequest } from '@/lib/request-origin';
+import { logError } from '@/lib/logger';
+
+export async function POST(request: NextRequest) {
+  try {
+    const limited = await rateLimit(request, 'mutate');
+    if (limited) return limited;
+
+    if (!isTrustedSameOriginRequest(request)) {
+      return apiErrors.forbidden('Invalid request origin');
+    }
+
+    const session = await auth();
+    if (!session?.user?.id) {
+      return apiErrors.unauthorized();
+    }
+
+    if (!isStripeFeatureEnabled()) {
+      return apiErrors.badRequest('Stripe billing is disabled by this host');
+    }
+
+    if (!isStripeConfigured()) {
+      return apiErrors.internalError('Stripe billing is not configured');
+    }
+
+    const billing = await getBillingOverview(session.user.id);
+    const customerId = billing.subscription.stripeCustomerId;
+    if (!customerId) {
+      return apiErrors.badRequest('No Stripe customer exists for this account');
+    }
+
+    const subscription = await findLiveStripeSubscription(customerId);
+    if (!subscription) {
+      return apiErrors.badRequest('No subscription to cancel');
+    }
+
+    const stripe = getStripe();
+    const unpaid = isUnpaidStripeSubscription(subscription);
+
+    // Scheduling an unpaid subscription to the end of its period leaves the customer
+    // owing money for a period they never paid for, while the already issued invoice
+    // keeps retrying their card on its own. Those cancel immediately instead, and the
+    // invoice for the unserved period is voided in the same pass.
+    const canceled = unpaid
+      ? await stripe.subscriptions.cancel(subscription.id)
+      : await stripe.subscriptions.update(subscription.id, { cancel_at_period_end: true });
+
+    const voidedInvoices = unpaid
+      ? await voidOpenSubscriptionInvoices(customerId, subscription.id)
+      : [];
+
+    await syncStripeSubscriptionToUser(canceled);
+
+    const response = successResponse({
+      canceledImmediately: unpaid,
+      status: canceled.status,
+      cancelAt: canceled.cancel_at ? new Date(canceled.cancel_at * 1000).toISOString() : null,
+      voidedInvoices,
+    });
+    return withCacheControl(response, 'private, no-store');
+  } catch (error) {
+    logError('Error canceling Stripe subscription:', error);
+    return apiErrors.internalError('Failed to cancel subscription');
+  }
+}

app/api/billing/checkout/route.ts

@@ -1,7 +1,11 @@
 import { NextRequest } from 'next/server';
 import { auth } from '@/lib/auth';
 import { apiErrors, successResponse, withCacheControl } from '@/lib/api-response';
-import { getOrCreateStripeCustomerId, getStripeCheckoutState } from '@/lib/billing';
+import {
+  findBlockingStripeSubscription,
+  getOrCreateStripeCustomerId,
+  getStripeCheckoutState,
+} from '@/lib/billing';
 import { rateLimit } from '@/lib/rate-limit';
 import { isStripeFeatureEnabled } from '@/lib/feature-flags';
 import { getStripe, getStripePriceId, isStripeConfigured } from '@/lib/stripe';
@@ -57,6 +61,17 @@ export async function POST(request: NextRequest) {
     const stripe = getStripe();
     const priceId = getStripePriceId();
     const customerId = await getOrCreateStripeCustomerId(session.user.id);
+
+    // The guard above reads the local mirror, which can be stale or cleared: the incident
+    // that prompted this had a customer holding three subscriptions at once because the
+    // mirror said there were none. Stripe is the one that knows.
+    const blockingSubscription = await findBlockingStripeSubscription(customerId);
+    if (blockingSubscription) {
+      return apiErrors.badRequest(
+        'A subscription already exists for this account. Manage it from the billing portal.'
+      );
+    }
+
     const appOrigin = getAppOrigin(request);
 
     const checkoutSession = await stripe.checkout.sessions.create({

app/api/billing/portal/route.ts

@@ -1,4 +1,5 @@
 import { NextRequest } from 'next/server';
+import type Stripe from 'stripe';
 import { auth } from '@/lib/auth';
 import { apiErrors, successResponse, withCacheControl } from '@/lib/api-response';
 import { getBillingOverview } from '@/lib/billing';
@@ -19,6 +20,40 @@ function getAppOrigin(request: NextRequest) {
   return request.nextUrl.origin;
 }
 
+async function readRequestedFlow(request: NextRequest) {
+  try {
+    const body = await request.json();
+    return body?.flow === 'payment_method_update' ? 'payment_method_update' : null;
+  } catch {
+    return null;
+  }
+}
+
+async function createPortalSession(
+  stripe: Stripe,
+  customer: string,
+  returnUrl: string,
+  flow: 'payment_method_update' | null
+) {
+  if (flow === 'payment_method_update') {
+    try {
+      return await stripe.billingPortal.sessions.create({
+        customer,
+        return_url: returnUrl,
+        flow_data: { type: 'payment_method_update' },
+      });
+    } catch (error) {
+      // The portal configuration may not expose this flow; the plain portal still works.
+      logError('Falling back to the default Stripe portal flow:', error);
+    }
+  }
+
+  return stripe.billingPortal.sessions.create({
+    customer,
+    return_url: returnUrl,
+  });
+}
+
 export async function POST(request: NextRequest) {
   try {
     const limited = await rateLimit(request, 'mutate');
@@ -47,10 +82,12 @@ export async function POST(request: NextRequest) {
     }
 
     const stripe = getStripe();
-    const portalSession = await stripe.billingPortal.sessions.create({
-      customer: billing.subscription.stripeCustomerId,
-      return_url: `${getAppOrigin(request)}/settings`,
-    });
+    const portalSession = await createPortalSession(
+      stripe,
+      billing.subscription.stripeCustomerId,
+      `${getAppOrigin(request)}/settings`,
+      await readRequestedFlow(request)
+    );
 
     const response = successResponse({ url: portalSession.url });
     return withCacheControl(response, 'private, no-store');

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,