Complete rate-limiter coverage for all background yt-dlp calls

- fetch_playlist_videos now uses _meta_run (background playlist indexing)
- fetch_channel_links now uses _meta_run (dead code, future-proofed)
- _upsert_video_from_yt accepts polite= flag; background enrichment passes polite=True
- Only intentional user-facing one-shot calls (download_subs_only,
  fetch_video_comments, polite=False metadata) bypass the lock

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

backend/services/ytdlp.py

@@ -435,7 +435,7 @@ def fetch_playlist_videos(playlist_id: str, max_videos: int = 200) -> list[dict]
     ]
     if max_videos > 0:
         args += ["--playlist-end", str(max_videos)]
-    stdout, _, code = _run(args, timeout=120)
+    stdout, _, code = _meta_run(args, timeout=120)
 
     videos = []
     for line in stdout.splitlines():
@@ -503,7 +503,7 @@ def fetch_channel_links(channel_id: str) -> list[str]:
         url = f"https://www.youtube.com/{channel_id}/about"
     else:
         url = f"https://www.youtube.com/channel/{channel_id}/about"
-    stdout, _, code = _run([
+    stdout, _, code = _meta_run([
         "yt-dlp",
         url,
         "--dump-json",