Admin: switch topics to tree editing layout

Render topics as a two-level tree so editing is clearer and limit updates to name/sort while parent selection stays create-only. Keep topic deletion safe with FK cascades and ensure photo tag links are cleaned automatically when a topic is removed.

lib/db_gallery.php

@@ -119,13 +119,6 @@ function topicUpdate(int $topicId, string $name, ?int $parentId, int $sortOrder
     $st->execute();
 }
 
-function topicChildrenCount(int $topicId): int
-{
-    $st = db()->prepare('SELECT COUNT(*) FROM topics WHERE parent_id=:id');
-    $st->execute(['id' => $topicId]);
-    return (int)$st->fetchColumn();
-}
-
 function topicDelete(int $topicId): void
 {
     $st = db()->prepare('DELETE FROM topics WHERE id=:id');