Restore embed-metadata/thumbnail for Jellyfin, drop wasteful faststart pass

The Merger+ffmpeg faststart postprocessor arg was overwritten by the
subsequent embed-metadata and embed-thumbnail passes anyway, making it
a pointless extra ffmpeg remux. Dropped it and restored the embeds.

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

backend/services/ytdlp.py

@@ -642,7 +642,7 @@ def start_download(
                     "yt-dlp", url,
                     "-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",