Add ffmpeg for HLS stream download support

- Add ffmpeg to Dockerfile.app for container HLS support
- Configure yt-dlp with --downloader ffmpeg --hls-use-mpegts
- Add startup health check warns if ffmpeg missing
- Update DEVELOPMENT.md with ffmpeg prerequisites and troubleshooting
- Add tests for ffmpeg HLS options and health check

Docker/Dockerfile.app

@@ -2,12 +2,13 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-# Install system dependencies for compiled Python packages
+# Install system dependencies for compiled Python packages and ffmpeg for HLS support
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
         gcc \
         g++ \
         libffi-dev \
+        ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Python dependencies (cached layer)