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>

tests/EditorSecurityTest.php

@@ -954,7 +954,7 @@ final class EditorSecurityTest extends WebTestCase
             $item = json_decode($client->getResponse()->getContent(), true)['item'];
             // Four steps now; the last one is capped at the width of the source (1800 here), so a
             // picture is never blown up beyond what was uploaded.
-            self::assertSame([480, 960, 1600, 1800], array_column($item['variants'], 'width'));
+            self::assertSame([480, 720, 960, 1280, 1600, 1800], array_column($item['variants'], 'width'));
             foreach ($item['variants'] as $variant) {
                 $file = $this->path.'-media/'.$item['id'].'-'.$variant['width'].'.webp';
                 self::assertSame('image/webp', getimagesize($file)['mime']);