Chore: Deprecate PAPERLESS_CONVERT_MEMORY_LIMIT and PAPERLESS_CONVERT_TMPDIR

These two options set ImageMagick's memory limit and scratch directory, but their only reader was the PDF thumbnail conversion helper. PDF thumbnails no longer use ImageMagick, so the settings did nothing while still being documented and advertised in the example configuration.

Remove the unused settings, add system warnings for anyone who still has either variable set, mark both options as deprecated and without effect in the configuration docs, and drop them from paperless.conf.example. The manual setup guide is also corrected so it no longer claims ImageMagick is needed for PDF conversion, and the ImageMagick policy step now only covers hardening.

docs/setup.md

@@ -177,7 +177,7 @@ 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
@@ -416,11 +416,11 @@ 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.
-    Paperless-ngx no longer passes PDF documents to ImageMagick, so enabling
-    PDF processing is not required.
+12. Harden ImageMagick by disabling formats that Paperless-ngx does not use.
+    Most distributions disable PDF processing by default, since PDF documents
+    can contain malware. Paperless-ngx no longer passes PDF documents to
+    ImageMagick, so enabling PDF processing is not required and should be left
+    disabled.
 
     Configure the active ImageMagick policy file (commonly
     `/etc/ImageMagick-6/policy.xml` or `/etc/ImageMagick-7/policy.xml`) and