feat(images): JPEG XL optimizer with webp/jpg fallback and <picture> serving

img_optimize() decodes .jxl with djxl or Imagick, resizes with GD, re-encodes
with cjxl and writes foto.jxl.webp beside it; skips cleanly without a decoder.
Gallery, hero, navbar logos and JSON-LD serve the JXL with the fallback, and
trash/removal move the fallback with its .jxl.

config/functions_events.php

@@ -225,6 +225,7 @@ function events_image_remove(?string $filename): void {
     if ($filename === '' || strncmp($filename, 'upl_', 4) !== 0) return;
     $path = events_image_dir() . $filename;
     if (is_file($path)) @unlink($path);
+    img_fallback_follow($path);
 }
 
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