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/PageController.php

@@ -532,7 +532,7 @@ final class PageController extends AbstractController
                         'errors' => [], 'values' => [],
                         'blockId' => $block['id'], 'page' => $page,
                         'issued' => $issued,
-                        'issuedSignature' => hash_hmac('sha256', (string) $issued, $this->secret),
+                        'issuedSignature' => FormRepository::stamp($slug, $issued, $this->secret),
                         // Leer, solange die Pruefung aus ist - dann steht auch kein Widget da.
                         'turnstileKey' => $this->turnstile->isActive() ? $this->turnstile->siteKey() : '',
                     ];