fix: install pandoc and enable pandoc conversions

Pandoc was not installed in the base image, so the export buttons
(docx, markdown, html, typst) were hidden because ENABLE_PANDOC_CONVERSIONS
defaulted to false.

- Dockerfile-base: add pandoc via apt (Ubuntu Noble ships 3.1.3, which
  supports --to typst added in pandoc 3.0)
- env.sh: set ENABLE_PANDOC_CONVERSIONS=true so both the web and CLSI
  services expose and serve the export endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server-ce/Dockerfile-base

@@ -25,6 +25,7 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
       unattended-upgrades \
       build-essential wget net-tools unzip time poppler-utils optipng strace nginx git python3 python-is-python3 zlib1g-dev libpcre3-dev gettext-base libwww-perl ca-certificates curl gnupg \
       qpdf \
+      pandoc \
 # upgrade base-image, batch all the upgrades together, rather than installing them on-by-one (which is slow!)
 &&  unattended-upgrade --verbose --no-minimal-upgrade-steps \
 # install Node.js https://github.com/nodesource/distributions#nodejs

server-ce/config/env.sh

@@ -1,3 +1,4 @@
+export ENABLE_PANDOC_CONVERSIONS=true
 export CHAT_HOST=127.0.0.1
 export CLSI_HOST=127.0.0.1
 export DOCSTORE_HOST=127.0.0.1