Speed up post-download merge by dropping embed-thumbnail and embed-metadata

Both flags trigger extra ffmpeg passes over the entire file after the
stream merge. They're unnecessary — metadata lives in the DB and
thumbnails come from YouTube. Removing them cuts the post-join wait
to just the faststart rewrite.

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

backend/services/ytdlp.py

@@ -643,7 +643,6 @@ def start_download(
                     "-f", fmt,
                     "--merge-output-format", "mp4",
                     "--postprocessor-args", "Merger+ffmpeg:-movflags +faststart",
-                    "--embed-metadata", "--embed-thumbnail",
                     "--no-part", "--no-mtime",
                     "-o", output_template,
                     "--newline", "--progress", "--no-colors",