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/Dockerfile

@@ -6,6 +6,13 @@ RUN apt-get update && apt-get install -y ffmpeg gosu && rm -rf /var/lib/apt/list
 
 COPY backend/requirements.txt ./backend/requirements.txt
 RUN pip install --no-cache-dir -r backend/requirements.txt && pip install --no-cache-dir -U yt-dlp
+
+# nodeenv downloads a pre-built Node.js binary via pip — no apt repos needed.
+# yt-dlp needs 'node' in PATH to solve YouTube's n-challenge (JS obfuscation).
+RUN pip install --no-cache-dir nodeenv && \
+    nodeenv --prebuilt /opt/node && \
+    ln -sf /opt/node/bin/node /usr/local/bin/node
+
 RUN echo "--cache-dir /data/yt-dlp-cache" > /etc/yt-dlp.conf
 
 COPY backend/ ./backend/