Add Node.js to Docker image; use web player client only

Node.js is required by yt-dlp to solve YouTube's n-challenge (format URL
deobfuscation). Without it the web client returns no video formats.

The tv and ios player clients were removed — both require GVS PO tokens
that we don't have, so they only produce warnings and block every request.
The web client with Node.js installed gives 30+ formats and works cleanly.

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

backend/Dockerfile

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