fix: subtitle embedding and MKV title not applied

postprocessor_args keys must be lowercase — cli_configuration_args does
key.lower() for lookup but the dict is case-sensitive, so "ffmpeg_o" and
"EmbedSubtitle+ffmpeg_o" never matched. Use "merger+ffmpeg_o" for title
during merge, "embedsubtitle+ffmpeg_o" for title+disposition during embed.

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

downloader.py

@@ -253,13 +253,14 @@ class DownloadManager:
                 finished_once[0] = True
                 self._set(dl_id, state="processing", progress=100)
 
-        ffmpeg_out = ["-metadata", f"title={mkv_title}"]
+        title_meta = ["-metadata", f"title={mkv_title}"]
 
         ydl_opts = {
             "outtmpl": f"{out_dir}/%(title)s.%(ext)s",
             "format": "bestvideo[vcodec^=avc1]+bestaudio/bestvideo+bestaudio/best",
             "merge_output_format": "mkv",
-            "postprocessor_args": {"ffmpeg_o": ffmpeg_out},
+            # "merger+ffmpeg_o" sets title during video+audio merge
+            "postprocessor_args": {"merger+ffmpeg_o": title_meta},
             "progress_hooks": [hook],
             "quiet": True,
             "no_warnings": True,
@@ -270,8 +271,10 @@ class DownloadManager:
                 "subtitleslangs": ["fr"],
                 "embedsubtitles": True,
             })
-            # Append disposition after title metadata
-            ffmpeg_out += ["-disposition:s:0", "default"]
+            # "embedsubtitle+ffmpeg_o" sets title + default disposition during subtitle embed
+            ydl_opts["postprocessor_args"]["embedsubtitle+ffmpeg_o"] = (
+                title_meta + ["-disposition:s:0", "default"]
+            )
 
         try:
             with yt_dlp.YoutubeDL(ydl_opts) as ydl: