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

src/server/fastapi_app.py

@@ -329,6 +329,19 @@ async def lifespan(_application: FastAPI):
         logger.info(
             "API documentation available at http://127.0.0.1:8000/api/docs"
         )
+
+        # Check for ffmpeg availability and warn if missing
+        try:
+            import shutil as _shutil
+            if _shutil.which("ffmpeg") is None:
+                logger.warning(
+                    "ffmpeg not found in PATH. HLS streams may fail to download. "
+                    "Install ffmpeg to enable HLS support."
+                )
+            else:
+                logger.debug("ffmpeg found at: %s", _shutil.which("ffmpeg"))
+        except Exception as _exc:
+            logger.warning("Could not check for ffmpeg: %s", _exc)
     except Exception as e:
         logger.error("Error during startup: %s", e, exc_info=True)
         startup_error = e