fix(feedback): sidebar links to /feedback/backlog/ not /feedback/

The submit endpoint only accepts POST; the sidebar was pointing to it
causing a 405 on click.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

vergabe_teilnahme/templates/partials/sidebar.html

@@ -46,7 +46,7 @@
     </div>
 
     <a href="/marktbegleiter/" class="sidebar-link">Marktbegleiter</a>
-    <a href="/feedback/" class="sidebar-link">Feedback-Backlog</a>
+    <a href="/feedback/backlog/" class="sidebar-link">Feedback-Backlog</a>
 
     {% if request.user.is_staff %}
     <a href="/admin/" class="sidebar-link text-slate-400">Administration</a>