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>

plugins/.htaccess

@@ -0,0 +1,9 @@
+# Dieser Ordner gehoert nicht ins Netz (ADR 116). Zeigt die Domain richtig auf public/, wird diese
+# Datei nie gelesen; zeigt sie versehentlich auf den Projektordner, sperrt sie hier alles.
+<IfModule mod_authz_core.c>
+    Require all denied
+</IfModule>
+<IfModule !mod_authz_core.c>
+    Order deny,allow
+    Deny from all
+</IfModule>