Session 45: support email, audit fixes (template validation, content-type, admin auth, waitUntil)

- Added support@docfast.dev to footer, impressum, terms, landing page, openapi.json
- Fixed audit #6: Template render validates required fields (400 on missing)
- Fixed audit #7: Content-Type check on markdown/URL routes (415)
- Fixed audit #11: /v1/usage and /v1/concurrency now require ADMIN_API_KEY
- Fixed audit Critical #3: URL convert uses domcontentloaded instead of networkidle0

templates/pages/index.html

@@ -407,7 +407,7 @@ html, body {
 <li>5,000 PDFs per month</li>
           <li>All conversion endpoints</li>
           <li>All templates included</li>
-          <li>Priority support</li>
+          <li>Priority support (<a href="mailto:support@docfast.dev">support@docfast.dev</a>)</li>
         </ul>
         <button class="btn btn-primary" style="width:100%" id="btn-checkout">Get Started →</button>
       </div>