Add delete button on the enemy detail page

CSRF-protected POST to /enemies/{id}/delete with a confirm. Any
combatants referencing the enemy keep their snapshotted stats (the FK
sets enemy_id null), so deleting a monster type won't disturb a fight.

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

templates/enemy/show.html.twig

@@ -11,7 +11,14 @@
             <h1 class="h3 mb-0">{{ enemy.name }}</h1>
             <span class="text-body-secondary fst-italic">{{ enemy.metaLine }}</span>
         </div>
-        <a href="{{ path('app_enemies_edit', {id: enemy.id}) }}" class="btn btn-outline-secondary btn-sm">Edit stats</a>
+        <div class="d-flex gap-2">
+            <a href="{{ path('app_enemies_edit', {id: enemy.id}) }}" class="btn btn-outline-secondary btn-sm">Edit stats</a>
+            <form method="post" action="{{ path('app_enemies_delete', {id: enemy.id}) }}"
+                  onsubmit="return confirm('Delete “{{ enemy.name|e('js') }}” from the enemy list?')">
+                <input type="hidden" name="_token" value="{{ csrf_token('delete-enemy-' ~ enemy.id) }}">
+                <button class="btn btn-outline-danger btn-sm">Delete</button>
+            </form>
+        </div>
     </div>
 
     <div class="row g-4">