Let plugins run their own PHP code

A plugin can now bring a plugin.php that registers hooks, scheduled tasks, its own tables, public pages and admin screens. Failures stay contained to the plugin; a form on a plugin page can be submitted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

templates/admin/plugin-page.html.twig

@@ -0,0 +1,19 @@
+{% extends 'admin/shell.html.twig' %}
+{% set nav = 'plugins' %}
+{% block title %}{{ page.title }}{% endblock %}
+{% block heading %}{{ page.title }}{% endblock %}
+{% block lead %}{{ plugin.title }} · <a href="{{ path('admin_plugins') }}">Zurück zu den Plugins</a>{% endblock %}
+{% block content %}
+{% if actions %}
+<div class="favicon-actions">
+    {% for action in actions %}
+    <form method="post" action="{{ path('admin_plugin_action', {name: plugin.name, key: action.key}) }}">
+        <input type="hidden" name="_csrf_token" value="{{ csrf_token('plugins') }}">
+        <input type="hidden" name="zurueck" value="{{ page.key }}">
+        <button class="secondary" type="submit">{{ action.label }}</button>
+    </form>
+    {% endfor %}
+</div>
+{% endif %}
+{% include page.template with data only %}
+{% endblock %}