fix: subtitle embedding broken via Python API

embedsubtitles:True is a CLI-only option — it registers FFmpegEmbedSubtitlePP
only via __init__.py's opts processing, not when using YoutubeDL() directly.
Must explicitly declare the postprocessor via the 'postprocessors' key.

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

downloader.py

@@ -269,7 +269,10 @@ class DownloadManager:
             ydl_opts.update({
                 "writesubtitles": True,
                 "subtitleslangs": ["fr"],
-                "embedsubtitles": True,
+                # embedsubtitles:True is CLI-only — must register the postprocessor explicitly
+                "postprocessors": [
+                    {"key": "FFmpegEmbedSubtitle", "already_have_subtitle": False},
+                ],
             })
             # "embedsubtitle+ffmpeg_o" sets title + default disposition during subtitle embed
             ydl_opts["postprocessor_args"]["embedsubtitle+ffmpeg_o"] = (