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_edit.html

@@ -1,9 +1,12 @@
 <div class="panel link-card" id="link-{{ link.id }}">
   <form hx-put="/links/{{ link.id }}" hx-target="#link-{{ link.id }}" hx-swap="outerHTML"
         hx-disabled-elt="button">
-    <div class="muted" style="font-size:.8rem; overflow:hidden; text-overflow:ellipsis; white-space:nowrap; margin-bottom:8px;">
+    <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:3px 0 10px;">
+      🕓 hinzugefügt am {{ link.created_at | datum }}
+    </div>
 
     <div class="field">
       <label>Titel</label>