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/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;