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>

templates/blocks/form.html.twig

@@ -17,7 +17,9 @@
     {# Bait for automated senders. Hidden from view, from assistive technology and from tab order. #}
     <div class="form-trap" aria-hidden="true"><label for="website-{{ blockId }}">Website</label><input id="website-{{ blockId }}" name="website" type="text" tabindex="-1" autocomplete="off"></div>
 
-    {% if errors is not empty %}
+    {% if errors._form is defined %}
+    <p class="form-error-summary" role="alert">{{ errors._form|kant_t }}</p>
+    {% elseif errors is not empty %}
     <p class="form-error-summary" role="alert">{{ (errors|length == 1 ? 'Bitte prüfe 1 Eingabe. Die betroffenen Felder sind unten gekennzeichnet.' : 'Bitte prüfe %d Eingaben. Die betroffenen Felder sind unten gekennzeichnet.')|kant_t|format(errors|length) }}</p>
     {% endif %}