Fix: Harden PDF thumbnail encoding and align CI and docs

Pillow raises DecompressionBombError, which is not an OSError, so an
enormous render from the unknown-geometry fallback DPI skipped the
default thumbnail fallback. It is now converted to a ParseError like other
encoding failures, with a test covering both error types.

CI did not install qpdf, which the new thumbnail repair tests need, so it
is added to the backend package list. The setup docs now list poppler-utils
as used for thumbnail generation and no longer claim PDF thumbnails fall
back to Ghostscript when the ImageMagick PDF policy is not enabled.

docs/setup.md

@@ -182,7 +182,7 @@ to a positive number to enable polling and disable native filesystem notificatio
     - `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:
 
@@ -419,8 +419,8 @@ to a positive number to enable polling and disable native filesystem notificatio
 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.
+    don't enable it, steps that still rely on ImageMagick, such as TIFF to
+    PDF conversion, may fail. PDF thumbnails no longer use ImageMagick.
 
     Configure the active ImageMagick policy file (commonly
     `/etc/ImageMagick-6/policy.xml` or `/etc/ImageMagick-7/policy.xml`) and