Real JSON null for empty product feature; SSR warm-cache button and CLI

Ticket 0068-FE: ProductFeatureJsonRenderer returned json_encode(null), which headless passes on as the STRING null (it only decodes object/array-looking strings) and crashed the front end. It now returns an empty string and the TypoScript field sets ifEmptyReturnNull = 1, giving a real JSON null. No other renderer used the pattern. Ticket 0066-FE: SsrPurgeService::warm() posts to the node /__warm endpoint (derived from SSR_PURGE_ENDPOINT, same secret), exposed as a Warm cache button with confirmation in the VITEC SSR module, a Cache warming progress row fed by /__health, and the CLI vitec:ssr-warm [paths]. Warm runs are deliberately not written to the purge log. Verified: two-path run 2/2 done, affected page delivers productfeature null.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

packages/vitec/Classes/Controller/Backend/SsrController.php

@@ -102,6 +102,22 @@ final class SsrController
         return new RedirectResponse((string)$this->uriBuilder->buildUriFromRoute('web_vitecssr'), 303);
     }
 
+    /**
+     * "Warm cache" button: asks the node to render every page of the
+     * sitemap in the background (0066-FE). Returns at once; progress is
+     * the "Cache warming" row of the status table.
+     */
+    public function warmAction(ServerRequestInterface $request): ResponseInterface
+    {
+        $result = $this->purgeService->warm();
+        $this->flash(
+            $result['message'],
+            $result['success'] ? 'Cache warming' : 'Cache warming failed',
+            $result['success']
+        );
+        return new RedirectResponse((string)$this->uriBuilder->buildUriFromRoute('web_vitecssr'), 303);
+    }
+
     /**
      * Ajax route vitec_ssr_purge_page: purge exactly one page, called by
      * the page tree context menu item (ssr-context-menu.js).