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>

tests/FormStepsTest.php

@@ -291,7 +291,7 @@ final class FormStepsTest extends TestCase
 
         $issued = time() - 10;
         $grund = [
-            'page' => 'home', 'block' => 'preis', 'issued' => (string) $issued, 'issuedSignature' => hash_hmac('sha256', (string) $issued, 'test-only'),
+            'page' => 'home', 'block' => 'preis', 'issued' => (string) $issued, 'issuedSignature' => FormRepository::stamp('preisfinder', $issued, 'test-only'),
             'immobilienart' => 'Grundstück', 'wohnflaeche' => '99', 'ausstattung' => ['Balkon'], 'flaeche' => '640', 'email' => 'x@example.org', 'datenschutz' => 'ja',
         ];
         $antwort = $this->request('/formular/preisfinder', 'POST', $grund);