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

tests/unit/test_ffmpeg_health_check.py

@@ -0,0 +1,54 @@
+"""Unit tests for ffmpeg health check in fastapi_app.py."""
+
+import asyncio
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+class TestFfmpegHealthCheck:
+    """Test ffmpeg health check warns when not in PATH."""
+
+    @pytest.mark.asyncio
+    async def test_ffmpeg_missing_warns(self):
+        """Should log warning when ffmpeg not found in PATH."""
+        with patch("shutil.which", return_value=None):
+            with patch("src.server.fastapi_app.setup_logging") as mock_log:
+                mock_logger = MagicMock()
+                mock_log.return_value = mock_logger
+
+                from src.server.fastapi_app import lifespan
+                app = MagicMock()
+
+                with pytest.raises(StopIteration):
+                    async with lifespan(app):
+                        pass
+
+                # Should have logged a warning about ffmpeg
+                warning_calls = [
+                    c for c in mock_logger.warning.call_args_list
+                    if "ffmpeg" in str(c)
+                ]
+                assert len(warning_calls) >= 1
+
+    @pytest.mark.asyncio
+    async def test_ffmpeg_present_no_warning(self):
+        """Should not log warning when ffmpeg is found."""
+        with patch("shutil.which", return_value="/usr/bin/ffmpeg"):
+            with patch("src.server.fastapi_app.setup_logging") as mock_log:
+                mock_logger = MagicMock()
+                mock_log.return_value = mock_logger
+
+                from src.server.fastapi_app import lifespan
+                app = MagicMock()
+
+                with pytest.raises(StopIteration):
+                    async with lifespan(app):
+                        pass
+
+                # Should NOT have logged a warning about ffmpeg
+                warning_calls = [
+                    c for c in mock_logger.warning.call_args_list
+                    if "ffmpeg" in str(c)
+                ]
+                assert len(warning_calls) == 0