fix(billing): preserve entitlements and bound cancellation cleanup

Integrate the latest cancellation-reason flow from master. Keep paid periods and independent trials intact, stop collection without erasing historical or mixed receivables, and make scheduled and partial cancellations recoverable. Add regression coverage for invoice boundaries, entitlement expiry, cancellation selection and concurrent reason writes.

app/api/billing/route.ts

@@ -1,7 +1,12 @@
 import { BillingSubscriptionStatus } from '@prisma/client';
 import { auth } from '@/lib/auth';
 import { apiErrors, successResponse, withCacheControl } from '@/lib/api-response';
-import { getBillingOverview, getOpenInvoiceForCustomer } from '@/lib/billing';
+import {
+  findCancelableStripeSubscription,
+  isUnpaidStripeSubscription,
+  getBillingOverview,
+  getOpenInvoiceForCustomer,
+} from '@/lib/billing';
 import { isStripeFeatureEnabled } from '@/lib/feature-flags';
 import { hasStripeRuntimeConfig, isStripeConfigured } from '@/lib/stripe';
 import { logError } from '@/lib/logger';
@@ -17,8 +22,7 @@ export async function GET() {
     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.
+    // Invoice details are only needed when the current subscription is behind on payment.
     const needsPaymentFix =
       billing.subscription.status === BillingSubscriptionStatus.PAST_DUE ||
       billing.subscription.status === BillingSubscriptionStatus.UNPAID;
@@ -30,6 +34,11 @@ export async function GET() {
           )
         : null;
 
+    const cancelable =
+      isStripeConfigured() && billing.subscription.stripeCustomerId
+        ? await findCancelableStripeSubscription(billing.subscription.stripeCustomerId)
+        : null;
+
     const response = successResponse({
       isEnabled,
       isConfigured,
@@ -42,20 +51,15 @@ export async function GET() {
         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,
+      // An already scheduled unpaid subscription still needs immediate cancellation.
+      // A different unscheduled subscription may also remain after an earlier cancel.
+      cancelAvailable: Boolean(cancelable),
       needsPaymentFix,
-      // Whether cancelling ends the subscription there and then rather than at the period
-      // end, which is what the confirmation copy has to say. Mirrors the branch the cancel
-      // route takes: nothing was paid for the open period, so there is nothing to run out.
-      cancelIsImmediate:
-        needsPaymentFix || billing.subscription.status === BillingSubscriptionStatus.INCOMPLETE,
+      cancelIsImmediate: Boolean(
+        cancelable &&
+        (isUnpaidStripeSubscription(cancelable) ||
+          ['canceled', 'incomplete_expired'].includes(cancelable.status))
+      ),
       openInvoice: openInvoice
         ? {
             id: openInvoice.id,