open-webui: add missing ffmpeg (#408685)

pkgs/by-name/op/open-webui/package.nix

@@ -5,6 +5,7 @@
   python312,
   nixosTests,
   fetchurl,
+  ffmpeg-headless,
 }:
 let
   pname = "open-webui";
@@ -39,6 +40,10 @@ let
         --replace-fail "npm run pyodide:fetch && vite build" "vite build"
     '';
 
+    propagatedBuildInputs = [
+      ffmpeg-headless
+    ];
+
     env.CYPRESS_INSTALL_BINARY = "0"; # disallow cypress from downloading binaries in sandbox
     env.ONNXRUNTIME_NODE_INSTALL_CUDA = "skip";
     env.NODE_OPTIONS = "--max-old-space-size=8192";