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>

.devcontainer/Dockerfile

@@ -39,7 +39,6 @@ ARG RUNTIME_PACKAGES="\
   ghostscript \
   gnupg \
   icc-profiles-free \
-  imagemagick \
   # PostgreSQL
   postgresql-client \
   # MySQL / MariaDB
@@ -93,21 +92,6 @@ RUN set -eux \
         https://github.com/paperless-ngx/builder/releases/download/jbig2enc-trixie-v${JBIG2ENC_VERSION}/jbig2enc_${JBIG2ENC_VERSION}-1_${TARGETARCH}.deb \
       && dpkg --install ./jbig2enc_${JBIG2ENC_VERSION}-1_${TARGETARCH}.deb
 
-# setup docker-specific things
-# These change sometimes, but rarely
-WORKDIR /usr/src/paperless/src/docker/
-
-COPY [ \
-  "docker/rootfs/etc/ImageMagick-6/paperless-policy.xml", \
-  "./" \
-]
-
-RUN set -eux \
-  && echo "Configuring ImageMagick" \
-    && mkdir -p /etc/ImageMagick-6 \
-    && mv paperless-policy.xml /etc/ImageMagick-6/policy.xml
-
-
 # Packages needed only for building a few quick Python
 # dependencies
 ARG BUILD_PACKAGES="\