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/cancel/route.ts

@@ -2,18 +2,25 @@ import { NextRequest } from 'next/server';
 import { auth } from '@/lib/auth';
 import { apiErrors, successResponse, withCacheControl } from '@/lib/api-response';
 import {
-  findLiveStripeSubscription,
-  getBillingOverview,
-  isUnpaidStripeSubscription,
-  syncStripeCustomerSubscriptions,
-  voidOpenSubscriptionInvoices,
-} from '@/lib/billing';
-import { rateLimit } from '@/lib/rate-limit';
+  CANCELLATION_NOTE_MAX_LENGTH,
+  cancelSubscription,
+  isCancellationReason,
+} from '@/lib/cancellation';
+import { RATE_LIMIT_CONFIGS, checkRateLimit, rateLimit, rateLimitHeaders } from '@/lib/rate-limit';
 import { isStripeFeatureEnabled } from '@/lib/feature-flags';
-import { getStripe, isStripeConfigured } from '@/lib/stripe';
+import { isStripeConfigured } from '@/lib/stripe';
 import { isTrustedSameOriginRequest } from '@/lib/request-origin';
 import { logError } from '@/lib/logger';
 
+/**
+ * In-app cancellation: end unpaid subscriptions immediately, schedule paid
+ * subscriptions for period end, and record the optional reason.
+ *
+ * This exists beside the Stripe portal rather than instead of it. The portal
+ * cannot ask a question of our own, and by the time its webhook arrives the
+ * customer has already left the page. Both fields are optional: skipping the
+ * question is allowed and must never stand between someone and cancelling.
+ */
 export async function POST(request: NextRequest) {
   try {
     const limited = await rateLimit(request, 'mutate');
@@ -28,6 +35,22 @@ export async function POST(request: NextRequest) {
       return apiErrors.unauthorized();
     }
 
+    // A second limit keyed on the account. The IP-keyed one above is shared by
+    // every mutating route and, without TRUSTED_PROXY_MODE, by every caller,
+    // so it is the wrong thing to lean on for the one action a leaving
+    // customer most needs to succeed.
+    const config = RATE_LIMIT_CONFIGS['billing-cancel'];
+    const limit = await checkRateLimit(session.user.id, 'billing-cancel', config);
+    if (!limit.allowed) {
+      return new Response(JSON.stringify({ error: 'Too many requests. Please try again later.' }), {
+        status: 429,
+        headers: {
+          'Content-Type': 'application/json',
+          ...rateLimitHeaders(limit, config.maxRequests),
+        },
+      });
+    }
+
     if (!isStripeFeatureEnabled()) {
       return apiErrors.badRequest('Stripe billing is disabled by this host');
     }
@@ -36,46 +59,55 @@ export async function POST(request: NextRequest) {
       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 body = await request.json().catch(() => null);
+    const rawReason = body?.reason ?? null;
+    if (rawReason !== null && !isCancellationReason(rawReason)) {
+      return apiErrors.badRequest('Unknown cancellation reason');
     }
 
-    const subscription = await findLiveStripeSubscription(customerId);
-    if (!subscription) {
-      return apiErrors.badRequest('No subscription to cancel');
+    const rawNote = body?.note;
+    if (rawNote !== undefined && rawNote !== null && typeof rawNote !== 'string') {
+      return apiErrors.badRequest('Note must be text');
+    }
+    const trimmedNote = typeof rawNote === 'string' ? rawNote.trim() : '';
+    if (trimmedNote.length > CANCELLATION_NOTE_MAX_LENGTH) {
+      return apiErrors.badRequest(
+        `Note must be at most ${CANCELLATION_NOTE_MAX_LENGTH} characters`
+      );
     }
 
-    const stripe = getStripe();
-    const unpaid = isUnpaidStripeSubscription(subscription);
+    const result = await cancelSubscription({
+      userId: session.user.id,
+      reason: rawReason,
+      note: trimmedNote.length > 0 ? trimmedNote : null,
+    });
 
-    // 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)
-      : [];
-
-    // Re-derived from the customer's whole set rather than written from `canceled` alone.
-    // A customer can hold more than one subscription, and mirroring just the one that was
-    // cancelled would lock out an account still being billed on another.
-    await syncStripeCustomerSubscriptions(customerId);
+    if (!result.ok) {
+      switch (result.code) {
+        case 'ALREADY_CANCELING':
+          return apiErrors.conflict(
+            'Your subscription is already set to end at the close of this period'
+          );
+        case 'STRIPE_REJECTED':
+          return apiErrors.conflict(
+            'Stripe could not find this subscription. Open Manage Subscription to see its current state.'
+          );
+        default:
+          return apiErrors.conflict('There is no active subscription to cancel');
+      }
+    }
 
     const response = successResponse({
-      canceledImmediately: unpaid,
-      status: canceled.status,
-      cancelAt: canceled.cancel_at ? new Date(canceled.cancel_at * 1000).toISOString() : null,
-      voidedInvoices,
+      cancelAtPeriodEnd: !result.canceledImmediately,
+      canceledImmediately: result.canceledImmediately,
+      status: result.status,
+      cancelAt: result.cancelAt?.toISOString() ?? null,
+      voidedInvoices: result.voidedInvoices,
+      periodEnd: result.periodEnd?.toISOString() ?? null,
     });
     return withCacheControl(response, 'private, no-store');
   } catch (error) {
-    logError('Error canceling Stripe subscription:', error);
+    logError('billing.cancel', error);
     return apiErrors.internalError('Failed to cancel subscription');
   }
 }