fix(video): transcode audio to AAC for MP4 streaming (PCM unsupported in MP4)

Co-authored-by: Copilot <[email protected]>

app.py

@@ -150,14 +150,15 @@ def local_video_file():
     except FileNotFoundError:
         return jsonify(error=f"Local video not found: {relative_path}"), 404
 
-    # Stream via ffmpeg: copy streams into an MP4 container with moov atom first
-    # so browsers can start playback immediately without downloading the whole file.
+    # Stream via ffmpeg: copy video, transcode audio to AAC (PCM not supported
+    # in MP4), output as fragmented MP4 so browser plays without full download.
     def generate_stream():
         cmd = [
             "ffmpeg", "-nostdin", "-loglevel", "error",
             "-i", path,
-            "-c", "copy",           # no re-encode, just remux
-            "-movflags", "frag_keyframe+empty_moov+faststart",
+            "-c:v", "copy",
+            "-c:a", "aac",
+            "-movflags", "frag_keyframe+empty_moov",
             "-f", "mp4",
             "pipe:1",
         ]