Opt-in WITH_FFMPEG build arg installs Debian's ffmpeg in the backend image

Off by default, since it adds about 400 MB. Documented with the clip recipe
and in the configuration reference.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/Dockerfile

@@ -26,6 +26,16 @@ ENV PATH="/app/.venv/bin:$PATH"
 # environment somebody already works in is the case that adopts instead.
 ENV NODE_VENV=managed
 
+# ffmpeg for node code that transcodes, such as the clip recipe in
+# docs/code/nodes.md. Off by default: Debian's package and what it pulls in add
+# about 400 MB. Set WITH_FFMPEG=true in .env and rebuild to include it.
+ARG WITH_FFMPEG=false
+RUN if [ "$WITH_FFMPEG" = true ]; then \
+      apt-get update \
+      && apt-get install -y --no-install-recommends ffmpeg \
+      && rm -rf /var/lib/apt/lists/*; \
+    fi
+
 # Install dependencies
 # Ref: https://docs.astral.sh/uv/guides/integration/docker/#intermediate-layers
 RUN --mount=type=cache,target=/root/.cache/uv \