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>

src/Plugin/PluginTask.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace App\Plugin;
+
+use App\Ops\Tasks\Task;
+
+/** Eine Aufgabe, die ein Plugin angemeldet hat, im Vertrag des Aufgabenplans. */
+final class PluginTask implements Task
+{
+    /** @param callable(): string $callback */
+    public function __construct(
+        private string $name,
+        private string $label,
+        private string $explanation,
+        private int $interval,
+        private $callback,
+    ) {
+    }
+
+    public function name(): string
+    {
+        return $this->name;
+    }
+
+    public function label(): string
+    {
+        return $this->label;
+    }
+
+    public function explanation(): string
+    {
+        return $this->explanation;
+    }
+
+    public function interval(): int
+    {
+        return $this->interval;
+    }
+
+    public function run(): string
+    {
+        return (string) ($this->callback)();
+    }
+}