Keep old pages readable, detect stale settings writes, make backups honest

- Own CSS is parsed on write only. A page whose stored CSS no longer
  passes a stricter rule answers 200 and opens in the editor; that one
  block loses its CSS. A document read several times in one request is
  validated once.
- Settings and theme refuse a write from an outdated form.
- A backup fails when a media file does not arrive completely and
  leaves no half directory behind; rotation counts intact backups only.
- The archive update takes its backup behind the maintenance page.
- A package import puts all media in place before removing the old
  ones, reports honestly when that fails and keeps a maintenance state
  that was already on.
- Restoring replaces the SQLite file in one move and points at pending
  migrations.
- Six irreversible actions ask first, in a dialog of the admin's own.
- Ctrl+S in the editor takes the field being typed in along; Delete
  no longer removes a block while a dialog is open.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

templates/admin/content-results.html.twig

@@ -170,7 +170,7 @@
                         {# The class is what the security test looks for: an editor must not even be
                            offered this. #}
                         <div class="delete-panel">
-                            <form action="{{ path('admin_delete', {id: page.id}) }}" method="post">
+                            <form action="{{ path('admin_delete', {id: page.id}) }}" method="post" data-confirm-title="„{{ page.title }}“ endgültig löschen?" data-confirm-text="Die Seite wird mit allen Fassungen gelöscht. Das lässt sich nicht rückgängig machen." data-confirm-label="Endgültig löschen" data-confirm-variant="danger">
                                 <input type="hidden" name="_list_type" value="{{ contentType }}"><input type="hidden" name="_list_query" value="{{ query }}"><input type="hidden" name="_list_status" value="{{ status }}"><input type="hidden" name="_list_page" value="{{ result.page }}"><input type="hidden" name="_list_limit" value="{{ result.limit }}">
         <input type="hidden" name="_csrf_token" value="{{ csrf_token('pages') }}">
                                 <input type="hidden" name="expectedVersion" value="{{ page.version }}">