Defer ffmpeg in the image; record the decision

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

backend/Dockerfile

@@ -2,12 +2,6 @@ FROM python:3.13
 
 ENV PYTHONUNBUFFERED=1
 
-# ffmpeg for node code, e.g. the docs' recipe that re-encodes a clip for a
-# wall panel that only decodes H.264 Baseline.
-RUN apt-get update \
-    && apt-get install -y --no-install-recommends ffmpeg \
-    && rm -rf /var/lib/apt/lists/*
-
 # Install uv
 # Ref: https://docs.astral.sh/uv/guides/integration/docker/#installing-uv
 COPY --from=ghcr.io/astral-sh/uv:0.9.26 /uv /uvx /bin/