Checkpoint: headless JSON architecture documented + cleanup (Stufe 1+2)

Restore point before Stufe 3 (central resolver-service refactoring).

Includes this session's work:
- Success Story (usecase) rebuild: new fields/tabs, UsecaseSerializer,
  thin List/Show renderers, MM relations, inline content elements.
- vitec_columns content block (two-column layout with per-item content)
  incl. unified header section; special-cased JSON resolution.
- Container per-column flex (items[].config align/justify) + gap on parent.
- Unified header section across CEs/plugins/containers; header fields in JSON.
- ISO-style architecture spec: Documentation/Headless-JSON-Architecture.md.
- Cleanup: removed local scratch + verified .bak backups.
- Fixes: B-6 (undefined thumbnail variable in Downloadcardcollection image
  resolver), B-7 (unsatisfiable legacy CType OR branch in Downloadcard/Datasheets).

Rollback: git reset --hard snapshot-2026-07-09-pre-stufe3

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/vitec/Classes/Controller/EventController.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Evomedien\Vitec\Controller;
+
+use Evomedien\Vitec\Domain\Repository\EventRepository;
+use Psr\Http\Message\ResponseInterface;
+use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;
+
+/**
+ * This file is part of the "VITEC" Extension for TYPO3 CMS.
+ *
+ * For the full copyright and license information, please read the
+ * LICENSE.txt file that was distributed with this source code.
+ *
+ * (c) 2026
+ */
+
+/**
+ * EventController — Extbase registration target for the Eventlist plugin.
+ * In headless mode the JSON output is produced by EventlistJsonRenderer,
+ * not by this controller.
+ */
+class EventController extends ActionController
+{
+    protected EventRepository $eventRepository;
+
+    public function __construct(EventRepository $eventRepository)
+    {
+        $this->eventRepository = $eventRepository;
+    }
+
+    public function listAction(): ResponseInterface
+    {
+        $this->view->assign('events', $this->eventRepository->findUpcoming());
+        return $this->htmlResponse();
+    }
+}