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/partials/footer.html

@@ -6,6 +6,7 @@
       <a href="/docs">Docs</a>
       <a href="/health">API Status</a>
       <a href="/#change-email" class="open-email-change">Change Email</a>
+      <a href="mailto:support@docfast.dev">Support</a>
       <a href="/impressum">Impressum</a>
       <a href="/privacy">Privacy Policy</a>
       <a href="/terms">Terms of Service</a>