fix: close remaining concurrent yt-dlp session sources

Three more code paths were bypassing the _meta_lock guard and firing
raw yt-dlp processes concurrently with active downloads:

- Popular fetch Phase 1 (flat-playlist channel crawl): changed from
  ytdlp._run to ytdlp._meta_run so it waits for active downloads
- download_subs_only: changed from _run to _meta_run
- fetch_video_comments: returns empty list immediately if a download
  is active (avoids blocking a 90s call indefinitely)
- Diagnostic test endpoint (settings): switched to _meta_run

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

backend/routers/channels.py

@@ -743,7 +743,7 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str, channel_name:
     else:
         url = f"https://www.youtube.com/channel/{youtube_channel_id}/videos"
 
-    stdout, _, _ = ytdlp._run([
+    stdout, _, _ = ytdlp._meta_run([
         "yt-dlp", url,
         "--dump-json", "--flat-playlist",
         "--quiet",