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>

scripts/editor-check.mjs

@@ -16,7 +16,7 @@ await new Promise(resolve => probe.close(resolve));
 const origin = `http://127.0.0.1:${port}`;
 const server = spawn(php, ['-d', 'upload_max_filesize=8M', '-d', 'post_max_size=10M', '-d', 'memory_limit=256M', '-S', `127.0.0.1:${port}`, '-t', 'public'], {
   windowsHide: true,
-  env: { ...process.env, APP_ENV: 'prod', APP_DEBUG: '0', APP_SECRET: randomBytes(32).toString('hex'), DATABASE_URL: fixture.databaseUrl, MEDIA_DIRECTORY: fixture.mediaDirectory, SITE_ORIGIN: origin },
+  env: { ...process.env, APP_ENV: 'prod', APP_DEBUG: '0', APP_SECRET: randomBytes(32).toString('hex'), DATABASE_URL: fixture.databaseUrl, MEDIA_DIRECTORY: fixture.mediaDirectory, MEDIA_MIRROR_DIRECTORY: fixture.mediaDirectory + '-abbild', SITE_ORIGIN: origin },
   stdio: 'ignore',
 });
 server.on('error', error => { throw error; });