Install Node.js via nodeenv (pip); use web client

nodeenv is a Python package that downloads a pre-built Node.js binary —
no apt repos, no compilation, guaranteed to work in python:3.12-slim.
The 'node' binary is linked into /usr/local/bin so yt-dlp can find it.

With Node.js available the web client works fully (37 formats) and can
solve YouTube's n-challenge that every other approach was failing on.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/settings.py

@@ -247,7 +247,7 @@ def ytdlp_test(
             "yt-dlp",
             "https://www.youtube.com/watch?v=dQw4w9WgXcQ",
             "--dump-json", "--no-download", "--no-playlist",
-            "--extractor-args", "youtube:player_client=web_embedded",
+            "--extractor-args", "youtube:player_client=web",
             *cookie_args,
         ],
         capture_output=True, text=True, timeout=30,