Switch to android_vr player client — no Node.js required

android_vr provides pre-signed format URLs that bypass YouTube's
n-challenge and signature JS requirements entirely. Tested: 23 video
formats available without any JavaScript runtime installed.

Reverts Node.js Dockerfile addition (which failed to build anyway).

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

backend/Dockerfile

@@ -2,9 +2,7 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-RUN apt-get update && apt-get install -y ffmpeg gosu nodejs && \
-    ln -sf /usr/bin/nodejs /usr/bin/node && \
-    rm -rf /var/lib/apt/lists/*
+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