Chore: Drop ImageMagik as a dependency (#14440)

* Mark the thumbnail constants Final

* Comment each pdftoppm option

* Feature/Chore: Replaces all usages of ImageMagik's convert binary with a simple Pillow based helper

This removes ImageMagik as a dependency entirly and all documentation is cleaned up from it.
We no longer need to install it, have a policy for it to allow PDFs and no longer shell
out to it either.

* Fix: Save WebP images losslessly when removing the alpha channel

Pillow's default WebP save is lossy, so a WebP with an alpha channel came
back from remove_alpha_channel with slightly shifted pixel values before
it reached OCR. Save WebP lossless; other formats are unchanged.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

paperless.conf.example

@@ -85,7 +85,3 @@ PAPERLESS_SECRET_KEY=change-me
 #PAPERLESS_TIKA_ENABLED=false
 #PAPERLESS_TIKA_ENDPOINT=http://localhost:9998
 #PAPERLESS_TIKA_GOTENBERG_ENDPOINT=http://localhost:3000
-
-# Binaries
-
-#PAPERLESS_CONVERT_BINARY=/usr/bin/convert