Owner settings: add a real delete for Mitarbeiter (barbers)

Rename + active-toggle already existed (this session, earlier commit)
-- this adds a genuine, permanent delete alongside them, guarded by a
confirm() prompt that points to the "Aktiv" checkbox as the reversible
alternative for someone just temporarily off. Existing bookings against
a deleted resource are left as-is (no FK in this schema, matching its
existing convention) -- the owner agenda already falls back to the raw
resource_id for a booking whose resource no longer resolves, same as
it does today for a deactivated one.

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

backoffice/app/templates/owner/settings.html

@@ -140,6 +140,12 @@
         <button type="submit" class="btn btn-small">Speichern</button>
       </form>
 
+      <form class="inline" method="post"
+            action="{{ url_for('owner_settings.delete_resource', resource_id=r.resource_id) }}"
+            onsubmit="return confirm('{{ r.name }} wirklich löschen? Das kann nicht rückgängig gemacht werden. Zum vorübergehenden Entfernen stattdessen oben „Aktiv&quot; abwählen.');">
+        <button type="submit" class="btn btn-danger btn-small">Mitarbeiter löschen</button>
+      </form>
+
       <form method="post" action="{{ url_for('owner_settings.update_resource_hours', resource_id=r.resource_id) }}">
         {% set hours = hours_by_resource.get(r.resource_id, {}) %}
         {% set day_names = {0: 'Montag', 1: 'Dienstag', 2: 'Mittwoch', 3: 'Donnerstag',