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/settings.py

@@ -250,7 +250,7 @@ def ytdlp_test(
     except Exception:
         pass
 
-    result = subprocess.run(
+    test_stdout, test_stderr, test_code = ytdlp._meta_run(
         [
             "yt-dlp",
             "https://www.youtube.com/watch?v=dQw4w9WgXcQ",
@@ -258,15 +258,15 @@ def ytdlp_test(
             "--extractor-args", "youtube:player_client=web",
             *cookie_args,
         ],
-        capture_output=True, text=True, timeout=30,
+        timeout=30,
     )
     return {
         "node_path": node_path,
         "node_version": node_version,
         "yt_dlp_version": yt_version,
         "cookie_args": cookie_args,
-        "returncode": result.returncode,
-        "stdout_lines": result.stdout.splitlines()[:5],
-        "stderr_tail": result.stderr.splitlines()[-20:],
-        "success": result.returncode == 0,
+        "returncode": test_code,
+        "stdout_lines": test_stdout.splitlines()[:5],
+        "stderr_tail": test_stderr.splitlines()[-20:],
+        "success": test_code == 0,
     }