Chore: Generate PDF thumbnails with pdftoppm and Pillow instead of ImageMagick

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>

docs/setup.md

@@ -177,12 +177,12 @@ to a positive number to enable polling and disable native filesystem notificatio
     - `pkg-config` for mysqlclient (python dependency)
     - `fonts-liberation` for generating thumbnails for plain text
       files
-    - `imagemagick` >= 6 for PDF conversion
+    - `imagemagick` >= 6 for image alpha handling
     - `gnupg` for decrypting GPG-encrypted email
     - `libpq-dev` for PostgreSQL
     - `libmagic-dev` for mime type detection
     - `mariadb-client` for MariaDB compile time
-    - `poppler-utils` for barcode detection
+    - `poppler-utils` for thumbnail generation and barcode detection
 
     Use this list for your preferred package management:
 
@@ -416,11 +416,8 @@ to a positive number to enable polling and disable native filesystem notificatio
         You may need to change the path in the files. Example:
         `ExecStart=/opt/paperless/.local/bin/celery --app paperless worker --loglevel INFO`
 
-12. Configure ImageMagick to allow processing of PDF documents and disable
-    formats that Paperless-ngx does not use. Most distributions disable PDF
-    processing by default, since PDF documents can contain malware. If you
-    don't enable it, Paperless-ngx will fall back to Ghostscript for certain
-    steps such as thumbnail generation.
+12. Harden ImageMagick by disabling formats that Paperless-ngx does not use.
+    PDF processing is not needed and should stay disabled.
 
     Configure the active ImageMagick policy file (commonly
     `/etc/ImageMagick-6/policy.xml` or `/etc/ImageMagick-7/policy.xml`) and