feat: add deno runtime for yt-dlp YouTube JS extraction

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

backend/Dockerfile

@@ -2,9 +2,12 @@ FROM python:3.12-slim
 
 # Install system dependencies
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    ffmpeg \
+    ffmpeg curl unzip \
     && rm -rf /var/lib/apt/lists/*
 
+# Install deno (required by yt-dlp for YouTube JS extraction)
+RUN curl -fsSL https://deno.land/install.sh | DENO_INSTALL=/usr/local sh
+
 WORKDIR /app
 
 # Install Python dependencies