feat: add dark mode, navigation, and Web Vitals tracking to marketplace

Backend:
- Simplify DatabaseConfig: remove effective_url property and project root finder
- Update to Pydantic v2 model_config (replace nested Config class)
- Add web_vitals router to main.py and __init__.py
- Fix ExplorerService datetime handling (ensure timezone-naive comparisons)
- Fix status_label extraction to handle both enum and string job states

Frontend (Marketplace):
- Add dark mode toggle with system preference detection

website/404.html

@@ -244,7 +244,7 @@
                     </a>
                 </div>
                 <div class="link-item">
-                    <a href="mailto:aitbc@bubuit.net">
+                    <a href="mailto:andreas.fleckl@bubuit.net">
                         <i class="fas fa-envelope"></i> Contact Support
                     </a>
                 </div>