Replace the ImageMagick convert based PDF thumbnail path with pdftoppm and
Pillow. Keeps thumbnails at 500px wide for small and odd sized pages, covers
the qpdf repair and double failure paths, removes the unused run_convert
helper, hardens thumbnail encoding, deprecates PAPERLESS_CONVERT_MEMORY_LIMIT
and PAPERLESS_CONVERT_TMPDIR, and corrects the related docs and CI.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>