fix(docker): add ffmpeg + nodejs for yt-dlp YouTube extraction

yt-dlp requires:
- ffmpeg for sub-conversion (--convert-subs srt) and any format mux
- A JS runtime for YouTube player decryption (deno or node)

Without these, every caption extraction attempt in the meet-kevin-watcher
container fails with 'ffmpeg not found' + 'No supported JavaScript runtime
could be found'. Adding both to the runtime stage of Dockerfile.service.

Size impact: ~50 MB.

docker/Dockerfile.service

@@ -37,7 +37,7 @@ RUN uv pip install --system --no-cache-dir --no-deps -e .
 # ---------------------------------------------------------------------------
 FROM python:3.12-slim
 
-RUN apt-get update && apt-get install -y --no-install-recommends curl && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends curl ffmpeg nodejs && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app