Add "truly done" ticket archiving with auto-unhide

- Let the user mark closed tickets as really done and hide them, so
  tickets closed prematurely by an agent stay visible until confirmed
- User-side "archived" flag stored in the DB (independent of Zammad),
  added via a schema migration on existing databases
- Hide archived tickets from the overview by default with an
  "erledigte einblenden" toggle; archive/unarchive from the ticket
  detail page and via a per-row quick action
- Poller automatically un-hides an archived ticket when new activity is
  detected, so nothing is missed

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/ticket.html

@@ -7,8 +7,19 @@
 <section class="panel">
   <div class="panel-head">
     <h2>#{{ ticket.number }} – {{ ticket.title }}</h2>
-    <a class="btn-sm" href="{{ zammad_url }}/#ticket/zoom/{{ ticket.id }}"
-       target="_blank" rel="noopener">in Zammad öffnen ↗</a>
+    <div class="head-tools">
+      {% if ticket.archived %}
+        <form method="post" action="{{ url_for('unarchive_ticket', ticket_id=ticket.id) }}">
+          <button class="btn-ghost" type="submit">↩ Wieder einblenden</button>
+        </form>
+      {% else %}
+        <form method="post" action="{{ url_for('archive_ticket', ticket_id=ticket.id) }}">
+          <button class="btn-ghost" type="submit" title="Als wirklich erledigt ablegen und ausblenden">✓ Wirklich erledigt</button>
+        </form>
+      {% endif %}
+      <a class="btn-sm" href="{{ zammad_url }}/#ticket/zoom/{{ ticket.id }}"
+         target="_blank" rel="noopener">in Zammad öffnen ↗</a>
+    </div>
   </div>
   <dl class="meta">
     <div><dt>Status</dt><dd><span class="state state-{{ ticket.state }}">{{ ticket.state }}</span></dd></div>