feat: add FFmpeg dependencies to system packages

Add FFmpeg and its development libraries to setup/system-packages script:
- ffmpeg: Main FFmpeg executable
- libavcodec-dev: Audio/video codec library
- libavformat-dev: Audio/video format library
- libavutil-dev: Utility library for FFmpeg
- libswscale-dev: Video scaling library

These libraries are required for torchcodec to function properly with
DiffRhythm audio generation. Also added FFmpeg version verification
after installation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

arty.yml

@@ -139,11 +139,23 @@ scripts:
       htop \
       tmux \
       net-tools \
-      davfs2
+      davfs2 \
+      ffmpeg \
+      libavcodec-dev \
+      libavformat-dev \
+      libavutil-dev \
+      libswscale-dev
 
     echo ""
     echo "✓ System packages installed successfully"
 
+    # Verify FFmpeg installation
+    if ffmpeg -version > /dev/null 2>&1; then
+      echo "✓ FFmpeg installed: $(ffmpeg -version | head -1 | cut -d' ' -f3)"
+    else
+      echo "❌ WARNING: FFmpeg not found"
+    fi
+
   setup/python-env: |
     echo "========================================="
     echo "  Setting Up Python Environment"