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/Ops/SitePackage.php

@@ -3,6 +3,7 @@
 namespace App\Ops;
 
 use App\Content\DocumentValidator;
+use App\Content\MediaMirror;
 use App\Domain\InvalidDocument;
 use App\Domain\InvalidPage;
 use Doctrine\DBAL\Connection;
@@ -49,6 +50,7 @@ final class SitePackage
         private Updater $updater,
         private string $root,
         private string $mediaDirectory,
+        private ?MediaMirror $mirror = null,
     ) {
     }
 
@@ -231,6 +233,8 @@ final class SitePackage
                     return $fail();
                 }
                 $step('Medien ersetzen', true, sprintf('%d Datei(en) übernommen.', $moved));
+                // Was der Webserver noch aus dem Abbild lieferte, gehört zur Website von vorher.
+                $this->mirror?->prune();
             } finally {
                 if (!$inWartung) {
                     $this->updater->maintenance(false);