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

docs/reference/configuration.md

@@ -100,6 +100,12 @@ to the backend. An empty value needs that proxy when the frontend and backend
 are served separately. See [facility automation](../getting-started/facility-automation.md#run-it-long-term)
 for deployment settings.
 
+## Image options
+
+`WITH_FFMPEG=true` in `.env` installs `ffmpeg` into the backend image on the
+next build, for node code that transcodes video. It is off by default because
+it adds about 400 MB.
+
 ## Environment
 
 | Variable | Default | Purpose |