Serve images without PHP, add the 720 and 1280 steps, trim two hot paths

- Image widths are 480, 720, 960, 1280, 1600 and 2400. A phone needed
  721 pixels and got 960. Existing images receive the new steps from
  a task; stored pages pick them up at render time, so no document is
  rewritten. Mobile backgrounds use 720.
- A variant served once is mirrored to public/media, and a rewrite rule
  lets the web server deliver it from there. var/media stays the source.
- The hero preload follows the image each device really shows.
- The post navigation reads the order and then two posts instead of
  up to 500 documents.

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

src/Controller/MediaController.php

@@ -4,6 +4,7 @@ namespace App\Controller;
 
 use App\Content\DocumentLibrary;
 use App\Content\MediaLibrary;
+use App\Content\MediaMirror;
 use App\Content\SiteSettings;
 use App\Content\VideoLibrary;
 use App\Domain\StaleWrite;
@@ -18,7 +19,7 @@ use Symfony\Component\Security\Csrf\CsrfTokenManagerInterface;
 
 final class MediaController extends AbstractController
 {
-    public function __construct(private MediaLibrary $media, private CsrfTokenManagerInterface $csrf, private SiteSettings $siteSettings, private DocumentLibrary $documents, private VideoLibrary $videos)
+    public function __construct(private MediaLibrary $media, private CsrfTokenManagerInterface $csrf, private SiteSettings $siteSettings, private DocumentLibrary $documents, private VideoLibrary $videos, private MediaMirror $mirror)
     {
     }
 
@@ -380,6 +381,9 @@ final class MediaController extends AbstractController
     public function serve(string $id, int $width): Response
     {
         $path = $this->media->file($id, $width) ?? throw $this->createNotFoundException();
+        // Diese Anfrage kam noch bei PHP an; die nächste nach demselben Bild beantwortet der
+        // Webserver selbst.
+        $this->mirror->publish($id, $width, $path);
 
         return new BinaryFileResponse($path, headers: ['Content-Type' => 'image/webp', 'X-Content-Type-Options' => 'nosniff', 'Cache-Control' => 'public, max-age=31536000, immutable', 'Content-Security-Policy' => "default-src 'none'"]);
     }