fix: Configure HTTPS domain and OAuth callback route

- Update configuration for production HTTPS domain (devden.hiddenden.cafe)
- Add nginx reverse proxy for /api and /auth routes to backend
- Create auth-callback.html to handle Microsoft Entra ID OAuth redirect
- Fix API_URL in script.js to use same origin (remove :8000 port)
- Add cache-busting query parameter (?v=2) to script.js
- Update .env.example with HTTPS requirements documentation

This resolves Azure Entra ID redirect URI mismatch and enables proper
OAuth authentication flow through the nginx frontend proxy.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

index.html

@@ -63,6 +63,6 @@
             <div id="debugOutput" style="margin-top: 10px; max-height: 200px; overflow-y: auto;"></div>
         </div>
 
-        <script src="script.js"></script>
+        <script src="script.js?v=2"></script>
     </body>
 </html>