Harden KantSites after the security review

- The installer only opens with a per-installation key that install.php
  creates and shows; the key is deleted with the first account.
- Lock files keep .env, the database, backups and logs out of reach when a
  domain points at the project folder instead of public/.
- Linked sections render by their own schema version, so an old section
  cannot carry unfiltered HTML into a newer page.
- Editors no longer move, trash or change areas and post data of published
  pages; a scheduled publication goes live with the revision that was
  scheduled (migration 20261012000100).
- Form stamps are bound to their form, IPv6 senders count per /64, and a
  flood per form or overall stops storing with a visible message.
- The consent redirect rejects control characters, update targets must look
  like 1.2.3, and own CSS cannot reach outside its block.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Controller/PartialController.php

@@ -77,7 +77,7 @@ final class PartialController extends AbstractController
                 $request->request->getString('slug') ?: $this->slugify($name),
                 $name,
                 // A shared area starts with one empty section, so there is something to click on.
-                ['schemaVersion' => 11, 'title' => '' === $name ? 'Bereich' : $name, 'description' => 'Wird an jeder Stelle gezeigt, die ihn einbindet.', 'blocks' => [
+                ['schemaVersion' => DocumentValidator::SCHEMA_VERSION, 'title' => '' === $name ? 'Bereich' : $name, 'description' => 'Wird an jeder Stelle gezeigt, die ihn einbindet.', 'blocks' => [
                     ['id' => 'b_'.bin2hex(random_bytes(8)), 'type' => 'section', 'layout' => ['tone' => 'paper', 'width' => 'normal'], 'children' => []],
                 ]],
             );