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/Configuration/RequestMiddlewares.php

@@ -46,5 +46,23 @@ return [
                 'typo3/cms-frontend/site',
             ],
         ],
+        // Self-healing sync of the SSR routing flag file against the
+        // Site Set switch vitec.ssr.enabled (no core event fires on
+        // settings save, hence per-request sync; cheap file_exists when
+        // nothing changed). Also registered for the backend below.
+        'vitec/sync-ssr-flag' => [
+            'target' => \Evomedien\Vitec\Middleware\SyncSsrFlag::class,
+            'after' => [
+                'typo3/cms-core/normalized-params-attribute',
+            ],
+        ],
+    ],
+    'backend' => [
+        'vitec/sync-ssr-flag' => [
+            'target' => \Evomedien\Vitec\Middleware\SyncSsrFlag::class,
+            'after' => [
+                'typo3/cms-core/normalized-params-attribute',
+            ],
+        ],
     ],
 ];