SSR switch: site setting vitec.ssr.enabled gates .htaccess proxy to SSR node

- Site set Vitecset: new bool setting vitec.ssr.enabled (default off,
  backend switch under Site Management > Settings > VITEC Settings)
- SsrFlagService derives var/vitec-ssr-on.flag from the setting;
  SyncSsrFlag middleware (frontend + backend) re-syncs per request
  because TYPO3 v14 fires no event on site settings save;
  vitec:ssr-flag CLI command for status/--sync
- public/.htaccess: flag-gated block proxies page requests [P] to
  https://vitec-ssr.evomedien.de (mod_proxy incl. https target verified
  on this host 2026-11-09), same exclusions as the SPA fallback block;
  completely inert while the flag file is absent
- Emergency off without TYPO3: delete var/vitec-ssr-on.flag
- Tested end to end: flag on -> pages proxied to VPS Caddy (502 while
  the SSR node is not deployed yet), /_json untouched; flag off ->
  SPA shell exactly as before

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/vitec/Classes/Middleware/SyncSsrFlag.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Evomedien\Vitec\Middleware;
+
+use Evomedien\Vitec\Service\SsrFlagService;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
+
+/**
+ * Self-healing sync of the SSR routing flag (see SsrFlagService).
+ *
+ * Registered for BOTH stacks: backend, so flipping the Site Set switch
+ * takes effect with the very next backend request (the module reload
+ * right after saving), and frontend, so /_json traffic repairs the flag
+ * even if nobody is logged into the backend. When flag and setting
+ * agree - the normal case - this costs a single file_exists() per
+ * request.
+ */
+final class SyncSsrFlag implements MiddlewareInterface
+{
+    public function __construct(private readonly SsrFlagService $ssrFlagService)
+    {
+    }
+
+    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
+    {
+        $this->ssrFlagService->syncQuietly();
+        return $handler->handle($request);
+    }
+}