fix(ml): install ffmpeg in ml-worker image for video frame sampling

extract_video_frames() calls ffprobe/ffmpeg, which were only installed
in the web Dockerfile. Videos route through the ml-worker for WD14 +
SigLIP inference, so the tool needs to exist there too.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile.ml

@@ -6,11 +6,13 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
     PIP_NO_CACHE_DIR=1 \
     ML_MODEL_DIR=/models
 
-# System deps: Pillow / image handling + psycopg2 build deps (binary wheel usually fine but libpq is needed at runtime on slim)
+# System deps: Pillow / image handling + psycopg2 runtime libpq + ffmpeg for
+# video frame sampling (WD14+SigLIP inference on videos calls ffprobe/ffmpeg).
 RUN apt-get update && apt-get install -y --no-install-recommends \
         libgl1 \
         libglib2.0-0 \
         libpq5 \
+        ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app