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>

---------

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

Dockerfile

@@ -129,7 +129,6 @@ ARG RUNTIME_PACKAGES="\
   ghostscript \
   gnupg \
   icc-profiles-free \
-  imagemagick \
   # PostgreSQL
   postgresql-client \
   # MySQL / MariaDB
@@ -167,8 +166,6 @@ RUN set -eux \
         https://github.com/paperless-ngx/builder/releases/download/jbig2enc-trixie-v${JBIG2ENC_VERSION}/jbig2enc_${JBIG2ENC_VERSION}-1_${TARGETARCH}.deb \
       && echo "Installing jbig2enc" \
         && dpkg --install ./jbig2enc_${JBIG2ENC_VERSION}-1_${TARGETARCH}.deb \
-      && echo "Configuring imagemagick" \
-        && cp /etc/ImageMagick-6/paperless-policy.xml /etc/ImageMagick-6/policy.xml \
       && echo "Cleaning up image layer" \
         && rm --force --verbose *.deb \
     && rm --recursive --force --verbose /var/lib/apt/lists/* \