feat: add opt-in save-before-playing setting backed by server-compiled single-file downloads

Dockerfile

@@ -14,10 +14,13 @@ FROM oven/bun:1-debian
 # ---- System dependencies ----
 # python3 is required by yt-dlp for some extraction paths
 # ca-certificates for HTTPS fetches from yt-dlp
+# ffmpeg lets yt-dlp merge bestvideo+bestaudio into a single mp4 for the
+# "Save before playing" download path (GET /api/download?mux=1)
 RUN apt-get update -qq && \
     apt-get install -y --no-install-recommends \
       curl \
       python3 \
+      ffmpeg \
       ca-certificates && \
     rm -rf /var/lib/apt/lists/*