Compact HE-AAC audio saves: server lane with fdkaac, shrink saved songs from Saved videos

Dockerfile

@@ -16,11 +16,15 @@ FROM oven/bun:1-debian
 # 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 && \
+# fdkaac (Debian contrib) encodes the compact HE-AAC audio phones can save;
+# ffmpeg's built-in AAC encoder only does AAC-LC.
+RUN sed -i 's/^Components: main$/Components: main contrib/' /etc/apt/sources.list.d/debian.sources && \
+    apt-get update -qq && \
     apt-get install -y --no-install-recommends \
       curl \
       python3 \
       ffmpeg \
+      fdkaac \
       ca-certificates && \
     rm -rf /var/lib/apt/lists/*