fix: downgrade instead of delete key on subscription cancel

- Replace revokeByCustomer with downgradeByCustomer in keys.ts
  - Sets tier='free' in cache and DB (UPDATE, not DELETE)
- Add isDocFastSubscription() product filter helper in billing.ts
  - Filters all subscription events by prod_TygeG8tQPtEAdE
- Handle customer.subscription.updated event
  - Downgrades on status=canceled/past_due/unpaid or cancel_at_period_end=true
- Handle customer.subscription.deleted with product filter
  - Downgrades to free (was incorrectly deleting the key)

Fixes revenue integrity bug: cancelled Pro subscribers kept Pro access.

src/routes/billing.ts

@@ -1,6 +1,6 @@
 import { Router, Request, Response } from "express";
 import Stripe from "stripe";
-import { createProKey, revokeByCustomer } from "../services/keys.js";
+import { createProKey, downgradeByCustomer } from "../services/keys.js";
 import logger from "../services/logger.js";
 
 function escapeHtml(s: string): string {
@@ -22,6 +22,29 @@ const router = Router();
 // Track provisioned session IDs to prevent duplicate key creation
 const provisionedSessions = new Set<string>();
 
+const DOCFAST_PRODUCT_ID = "prod_TygeG8tQPtEAdE";
+
+// Returns true if the given Stripe subscription contains a DocFast product.
+// Used to filter webhook events — this Stripe account is shared with other projects.
+async function isDocFastSubscription(subscriptionId: string): Promise<boolean> {
+  try {
+    const sub = await getStripe().subscriptions.retrieve(subscriptionId, {
+      expand: ["items.data.price.product"],
+    });
+    return sub.items.data.some((item) => {
+      const price = item.price as Stripe.Price | null;
+      const productId =
+        typeof price?.product === "string"
+          ? price.product
+          : (price?.product as Stripe.Product | null)?.id;
+      return productId === DOCFAST_PRODUCT_ID;
+    });
+  } catch (err: any) {
+    logger.error({ err, subscriptionId }, "isDocFastSubscription: failed to retrieve subscription");
+    return false;
+  }
+}
+
 // Create a Stripe Checkout session for Pro subscription
 router.post("/checkout", async (_req: Request, res: Response) => {
   try {
@@ -126,7 +149,6 @@ router.post("/webhook", async (req: Request, res: Response) => {
       const email = session.customer_details?.email;
 
       // Filter by product — this Stripe account is shared with other projects
-      const DOCFAST_PRODUCT_ID = "prod_TygeG8tQPtEAdE";
       try {
         const fullSession = await getStripe().checkout.sessions.retrieve(session.id, {
           expand: ["line_items"],
@@ -156,11 +178,36 @@ router.post("/webhook", async (req: Request, res: Response) => {
       logger.info({ email, customerId }, "checkout.session.completed: provisioned pro key");
       break;
     }
+    case "customer.subscription.updated": {
+      const sub = event.data.object as Stripe.Subscription;
+      const customerId = sub.customer as string;
+      const shouldDowngrade =
+        sub.status === "canceled" ||
+        sub.status === "past_due" ||
+        sub.status === "unpaid" ||
+        sub.cancel_at_period_end === true;
+
+      if (shouldDowngrade) {
+        if (!(await isDocFastSubscription(sub.id))) {
+          logger.info({ subscriptionId: sub.id }, "customer.subscription.updated: ignoring event for different product");
+          break;
+        }
+        await downgradeByCustomer(customerId);
+        logger.info({ customerId, status: sub.status, cancelAtPeriodEnd: sub.cancel_at_period_end },
+          "customer.subscription.updated: downgraded key to free tier");
+      }
+      break;
+    }
     case "customer.subscription.deleted": {
       const sub = event.data.object as Stripe.Subscription;
       const customerId = sub.customer as string;
-      await revokeByCustomer(customerId);
-      logger.info({ customerId }, "Subscription cancelled, key revoked");
+
+      if (!(await isDocFastSubscription(sub.id))) {
+        logger.info({ subscriptionId: sub.id }, "customer.subscription.deleted: ignoring event for different product");
+        break;
+      }
+      await downgradeByCustomer(customerId);
+      logger.info({ customerId }, "customer.subscription.deleted: downgraded key to free tier");
       break;
     }
     default:

src/services/keys.ts

@@ -108,12 +108,11 @@ export async function createProKey(email: string, stripeCustomerId: string): Pro
   return entry;
 }
 
-export async function revokeByCustomer(stripeCustomerId: string): Promise<boolean> {
-  const idx = keysCache.findIndex((k) => k.stripeCustomerId === stripeCustomerId);
-  if (idx >= 0) {
-    const key = keysCache[idx].key;
-    keysCache.splice(idx, 1);
-    await pool.query("DELETE FROM api_keys WHERE key = $1", [key]);
+export async function downgradeByCustomer(stripeCustomerId: string): Promise<boolean> {
+  const entry = keysCache.find((k) => k.stripeCustomerId === stripeCustomerId);
+  if (entry) {
+    entry.tier = "free";
+    await pool.query("UPDATE api_keys SET tier = 'free' WHERE stripe_customer_id = $1", [stripeCustomerId]);
     return true;
   }
   return false;