Show the date a link was added

The created_at timestamp was already stored but never displayed. Add
a "datum" Jinja filter that converts the stored UTC value to local
time and render it on both the link card and the edit form.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/_link_card.html

@@ -6,6 +6,9 @@
       <div class="muted" style="font-size:.8rem; overflow:hidden; text-overflow:ellipsis; white-space:nowrap;">
         {{ link.url }}
       </div>
+      <div class="muted" style="font-size:.75rem; margin-top:3px;">
+        🕓 hinzugefügt am {{ link.created_at | datum }}
+      </div>
     </div>
     <div style="display:flex; gap:4px; flex-shrink:0;">
       <button class="ghost" style="padding:4px 10px; font-size:.8rem;"