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/Sets/Vitecset/settings.definitions.yaml

@@ -132,3 +132,10 @@ settings:
     category: seo
     type: string
     default: ''
+
+  vitec.ssr.enabled:
+    label: 'SSR delivery (serve pages from the SSR node)'
+    description: 'ON: page requests are proxied by .htaccess to the SSR server (React Router SSR on the Solr VPS) - the flag file var/vitec-ssr-on.flag is kept in sync automatically within seconds. OFF: pages are served as today (SPA shell from _frontend/). JSON under /_json, /typo3, /api etc. are never affected. Safe rollback switch.'
+    category: VITEC
+    type: bool
+    default: false