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/configuration.md

@@ -1315,29 +1315,17 @@ valid crontab(5) expression describing when to run.
 
 #### [`PAPERLESS_CONVERT_MEMORY_LIMIT=<num>`](#PAPERLESS_CONVERT_MEMORY_LIMIT) {#PAPERLESS_CONVERT_MEMORY_LIMIT}
 
-: On smaller systems, or even in the case of Very Large Documents, the
-consumer may explode, complaining about how it's "unable to extend
-pixel cache". In such cases, try setting this to a reasonably low
-value, like 32. The default is to use whatever is necessary to do
-everything without writing to disk, and units are in megabytes.
+!!! warning
 
-    For more information on how to use this value, you should search the
-    web for "MAGICK_MEMORY_LIMIT".
-
-    Defaults to 0, which disables the limit.
+    Deprecated and has no effect, since PDF thumbnails no longer use
+    ImageMagick. It will be removed in a future release.
 
 #### [`PAPERLESS_CONVERT_TMPDIR=<path>`](#PAPERLESS_CONVERT_TMPDIR) {#PAPERLESS_CONVERT_TMPDIR}
 
-: Similar to the memory limit, if you've got a small system and your
-OS mounts /tmp as tmpfs, you should set this to a path that's on a
-physical disk, like /home/your_user/tmp or something. ImageMagick
-will use this as scratch space when crunching through very large
-documents.
+!!! warning
 
-    For more information on how to use this value, you should search the
-    web for "MAGICK_TMPDIR".
-
-    Default is none, which disables the temporary directory.
+    Deprecated and has no effect, since PDF thumbnails no longer use
+    ImageMagick. It will be removed in a future release.
 
 #### [`PAPERLESS_APPS=<string>`](#PAPERLESS_APPS) {#PAPERLESS_APPS}