fix(billing): prevent duplicate subscriptions and make webhook sync authoritative

A Stripe customer can own several subscriptions. Two defects let that happen
and corrupt the user's billing state:

1. Checkout allowed a fresh subscription whenever the user was not ACTIVE/
   TRIALING, so a PAST_DUE user started a brand-new subscription (Stripe
   Checkout always creates one) instead of recovering the existing one.
   Add hasRecoverableSubscription() (ACTIVE/TRIALING/PAST_DUE/UNPAID/
   INCOMPLETE); block checkout and route these users to the billing portal
   ('Update Payment Method') both in the API guard and the settings UI.

2. Subscription webhooks trusted the event's single subscription, so an old
   subscription's deletion could clobber a newer active one (marking the user
   CANCELED / No access). Every subscription event now re-derives state from
   the full set of the customer's Stripe subscriptions via
   syncStripeCustomerSubscriptions() + selectAuthoritativeSubscription(),
   making the sync order-independent and self-healing.

lib/billing.ts

@@ -10,6 +10,18 @@ const ACTIVE_SUBSCRIPTION_STATUSES = new Set<BillingSubscriptionStatus>([
   BillingSubscriptionStatus.TRIALING,
 ]);
 
+// Statuses that mean the customer already has a live Stripe subscription that
+// should be recovered (via the billing portal / dunning) rather than duplicated
+// with a fresh checkout. Everything else (FREE, CANCELED, INCOMPLETE_EXPIRED)
+// has no recoverable subscription, so a new checkout is appropriate.
+const RECOVERABLE_SUBSCRIPTION_STATUSES = new Set<BillingSubscriptionStatus>([
+  BillingSubscriptionStatus.ACTIVE,
+  BillingSubscriptionStatus.TRIALING,
+  BillingSubscriptionStatus.PAST_DUE,
+  BillingSubscriptionStatus.UNPAID,
+  BillingSubscriptionStatus.INCOMPLETE,
+]);
+
 export const DEFAULT_TRIAL_PERIOD_DAYS = 7;
 const STORAGE_CLEANUP_GRACE_DAYS = 15;
 
@@ -35,6 +47,14 @@ export function hasActiveSubscription(status: BillingSubscriptionStatus | null |
   return ACTIVE_SUBSCRIPTION_STATUSES.has(status);
 }
 
+// True when the customer already has a live subscription (active/trialing OR a
+// recoverable one like past_due/unpaid/incomplete). Used to route them to the
+// billing portal instead of letting a new checkout create a duplicate.
+export function hasRecoverableSubscription(status: BillingSubscriptionStatus | null | undefined) {
+  if (!status) return false;
+  return RECOVERABLE_SUBSCRIPTION_STATUSES.has(status);
+}
+
 export function hasBillingAccess(subject: BillingAccessSubject, now: Date = new Date()) {
   if (!isStripeFeatureEnabled()) {
     return true;
@@ -170,6 +190,7 @@ export async function getStripeCheckoutState(userId: string) {
 
   return {
     hasActiveSubscription: hasActiveSubscription(user.subscriptionStatus),
+    hasRecoverableSubscription: hasRecoverableSubscription(user.subscriptionStatus),
     isTrialEligible: !user.billingTrialConsumedAt,
   };
 }
@@ -253,6 +274,7 @@ export async function getWorkspaceCreationEligibility(userId: string) {
       status: user.subscriptionStatus,
       label: getBillingStatusLabel(user.subscriptionStatus),
       hasActiveSubscription: hasActiveSubscription(user.subscriptionStatus),
+      hasRecoverableSubscription: hasRecoverableSubscription(user.subscriptionStatus),
       hasActiveTrial: hasActiveTrial(user.trialEndsAt),
       hasBillingAccess: billingAccess,
       isTrialEligible: !user.billingTrialConsumedAt,
@@ -410,6 +432,67 @@ export async function syncStripeSubscriptionToUser(subscription: Stripe.Subscrip
   });
 }
 
+// A single Stripe customer can own several subscriptions at once (e.g. after
+// going past_due and re-subscribing). Higher priority = more authoritative for
+// deciding the user's entitlement.
+const SUBSCRIPTION_STATUS_PRIORITY: Record<Stripe.Subscription.Status, number> = {
+  active: 100,
+  trialing: 90,
+  past_due: 80,
+  unpaid: 70,
+  paused: 60,
+  incomplete: 50,
+  incomplete_expired: 20,
+  canceled: 10,
+};
+
+// Picks the subscription that should drive the user's billing state when a
+// customer has more than one. Prefers subscriptions that carry the entitled
+// price, then the most "alive" status, then the most recently created.
+export function selectAuthoritativeSubscription(
+  subscriptions: Stripe.Subscription[]
+): Stripe.Subscription | null {
+  if (subscriptions.length === 0) {
+    return null;
+  }
+
+  return [...subscriptions].sort((a, b) => {
+    const aEntitled = Boolean(getEntitledStripePriceId(a));
+    const bEntitled = Boolean(getEntitledStripePriceId(b));
+    if (aEntitled !== bEntitled) {
+      return aEntitled ? -1 : 1;
+    }
+
+    const aStatus = SUBSCRIPTION_STATUS_PRIORITY[a.status] ?? 0;
+    const bStatus = SUBSCRIPTION_STATUS_PRIORITY[b.status] ?? 0;
+    if (aStatus !== bStatus) {
+      return bStatus - aStatus;
+    }
+
+    return (getStripeTimestamp(b.created) ?? 0) - (getStripeTimestamp(a.created) ?? 0);
+  })[0];
+}
+
+// Source-of-truth sync: instead of trusting a single subscription from a webhook
+// event body (which may be an OLD subscription being deleted while a NEWER one is
+// active), re-list ALL of the customer's subscriptions from Stripe and sync the
+// authoritative one. This is order-independent and self-healing.
+export async function syncStripeCustomerSubscriptions(customerId: string) {
+  const stripe = getStripe();
+  const { data: subscriptions } = await stripe.subscriptions.list({
+    customer: customerId,
+    status: 'all',
+    limit: 100,
+  });
+
+  const authoritative = selectAuthoritativeSubscription(subscriptions);
+  if (!authoritative) {
+    return markSubscriptionCanceledByCustomerId(customerId);
+  }
+
+  return syncStripeSubscriptionToUser(authoritative);
+}
+
 export async function markSubscriptionCanceledByCustomerId(
   customerId: string,
   options?: { currentPeriodEnd?: Date | null; endedAt?: Date | null }