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

docker/compose.yml

@@ -163,6 +163,9 @@ services:
     build:
       context: ..
       dockerfile: backend/Dockerfile
+      args:
+        # Opt-in: ffmpeg costs the image about 400 MB (backend/Dockerfile).
+        - WITH_FFMPEG=${WITH_FFMPEG:-false}
     labels:
       - autoheal=true
       - traefik.enable=true