Fix EJS: install yt-dlp[default] and enable --js-runtimes node

yt-dlp's EJS (External JavaScript Solver) needs two things:
1. The solver scripts — only bundled with yt-dlp[default], not bare yt-dlp
2. An explicit --js-runtimes flag — Node.js is not the default (Deno is)

Both are now set: pip installs the [default] extras, and /etc/yt-dlp.conf
sets --js-runtimes node globally so every yt-dlp call uses it.

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

backend/Dockerfile

@@ -5,15 +5,15 @@ WORKDIR /app
 RUN apt-get update && apt-get install -y ffmpeg gosu && rm -rf /var/lib/apt/lists/*
 
 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
+# yt-dlp[default] includes the EJS challenge solver scripts required by YouTube.
-
+# nodeenv installs Node.js (v22+ required); --js-runtimes node tells yt-dlp to use it.
-# nodeenv downloads a pre-built Node.js binary via pip — no apt repos needed.
+RUN pip install --no-cache-dir -r backend/requirements.txt && \
-# yt-dlp needs 'node' in PATH to solve YouTube's n-challenge (JS obfuscation).
+    pip install --no-cache-dir -U "yt-dlp[default]" && \
-RUN pip install --no-cache-dir nodeenv && \
+    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
+RUN printf -- '--cache-dir /data/yt-dlp-cache\n--js-runtimes node\n' > /etc/yt-dlp.conf
 
 COPY backend/ ./backend/