fix(backend): YouTube downloads — install Deno + pull yt-dlp nightly

Downloads were 403ing on two fronts: (1) recent yt-dlp needs a JS runtime to
solve YouTube's signature/nsig challenge (Deno, added in cb7a3d7), and (2) the
current yt-dlp *stable* had a bug that 403'd the android_vr client on every video
(yt-dlp#17456, fixed in nightly). Switch the backend from the stable yt-dlp
binary to nightly via pip (`--pre 'yt-dlp[default]'`). Verified end-to-end incl.
a real frontend download. No cookies / PO-token service / proxy needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -36,7 +36,7 @@ services:
     command: >
       sh -c "
         apt-get update && apt-get install -y --no-install-recommends python3 python3-pip ffmpeg curl git unzip && rm -rf /var/lib/apt/lists/* &&
-        curl -sL https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp -o /usr/local/bin/yt-dlp && chmod a+rx /usr/local/bin/yt-dlp &&
+        pip install --break-system-packages -U --pre 'yt-dlp[default]' &&
         curl -fsSL https://deno.land/install.sh | sh &&
         pip install --break-system-packages spotapi 'git+https://github.com/TzurSoffer/spotify-downloader' &&
         mkdir -p /deps && cd /deps && npm install express kuromoji &&