feat: convert change-email from modal to standalone page + Stripe customer.updated webhook

- Add /change-email as a proper standalone page (public/src/change-email.html)
  with API key input, new email input, verification code flow, and success state
- Update footer partial: change "/#change-email" link to "/change-email" on all pages
- Remove email change modal HTML and hash-handler JS from index page source
- Add /change-email to sitemap.xml
- Rebuild all HTML files via build-html.cjs

- Add updateEmailByCustomer() to src/services/keys.ts
- Add customer.updated webhook handler in src/routes/billing.ts
  to sync email changes made via Stripe dashboard back to DocFast

src/routes/billing.ts

@@ -1,6 +1,6 @@
 import { Router, Request, Response } from "express";
 import Stripe from "stripe";
-import { createProKey, downgradeByCustomer } from "../services/keys.js";
+import { createProKey, downgradeByCustomer, updateEmailByCustomer } from "../services/keys.js";
 import logger from "../services/logger.js";
 
 function escapeHtml(s: string): string {
@@ -210,6 +210,18 @@ router.post("/webhook", async (req: Request, res: Response) => {
       logger.info({ customerId }, "customer.subscription.deleted: downgraded key to free tier");
       break;
     }
+    case "customer.updated": {
+      const customer = event.data.object as Stripe.Customer;
+      const customerId = customer.id;
+      const newEmail = customer.email;
+      if (customerId && newEmail) {
+        const updated = await updateEmailByCustomer(customerId, newEmail);
+        if (updated) {
+          logger.info({ customerId, newEmail }, "Customer email synced from Stripe");
+        }
+      }
+      break;
+    }
     default:
       break;
   }

src/services/keys.ts

@@ -129,3 +129,11 @@ export async function updateKeyEmail(apiKey: string, newEmail: string): Promise<
   await pool.query("UPDATE api_keys SET email = $1 WHERE key = $2", [newEmail, apiKey]);
   return true;
 }
+
+export async function updateEmailByCustomer(stripeCustomerId: string, newEmail: string): Promise<boolean> {
+  const entry = keysCache.find(k => k.stripeCustomerId === stripeCustomerId);
+  if (!entry) return false;
+  entry.email = newEmail;
+  await pool.query(UPDATE api_keys SET email = $1 WHERE stripe_customer_id = $2, [newEmail, stripeCustomerId]);
+  return true;
+}