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

public/partials/_footer.html

@@ -5,7 +5,7 @@
       <a href="/">Home</a>
       <a href="/docs">Docs</a>
       <a href="/status">API Status</a>
-      <a href="/#change-email" class="open-email-change">Change Email</a>
+      <a href="/change-email">Change Email</a>
       <a href="/impressum">Impressum</a>
       <a href="/privacy">Privacy Policy</a>
       <a href="/terms">Terms of Service</a>