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/TaskScreenTest.php

@@ -78,7 +78,7 @@ final class TaskScreenTest extends WebTestCase
         self::assertStringContainsString('Ohne Besuch und ohne geöffnete Verwaltung passiert nichts.', $html);
         self::assertStringContainsString('/cms-cron/'.Scheduler::cronToken(self::SECRET), $html);
         // Und der Plan steht danach in der Datenbank - eine Zeile je Aufgabe, nicht mehr.
-        self::assertSame(6, (int) $this->db->fetchOne('SELECT COUNT(*) FROM scheduled_task'));
+        self::assertSame(7, (int) $this->db->fetchOne('SELECT COUNT(*) FROM scheduled_task'));
     }
 
     public function testStartingWorkNeedsAnAdministratorAndAValidForm(): void