fix(backend): install Deno so yt-dlp can solve YouTube's signature/nsig challenge

Recent yt-dlp (which the backend re-pulls latest on every restart) needs an
external JavaScript runtime to solve YouTube's signature/nsig challenge — without
it, "Signature solving failed" → formats get skipped/HTTP 403. Install Deno in the
backend container (+ unzip for its installer) and put it on PATH for the node
process so spawned yt-dlp finds it. No extra container, no account, no proxy.

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

docker-compose.yml

@@ -35,11 +35,12 @@ services:
       - TICKETMASTER_API_KEY=${TICKETMASTER_API_KEY}
     command: >
       sh -c "
-        apt-get update && apt-get install -y --no-install-recommends python3 python3-pip ffmpeg curl git && rm -rf /var/lib/apt/lists/* &&
+        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 &&
+        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 &&
-        cd /app && NODE_PATH=/deps/node_modules node novasonic-server.js
+        cd /app && PATH=/root/.deno/bin:$$PATH NODE_PATH=/deps/node_modules node novasonic-server.js
       "
 
   # ── Scrobble service (isolated timestamped play-log → stats + Wrapped) ────────