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/routers/videos.py

@@ -513,9 +513,9 @@ def _row_to_detail(row) -> VideoDetail:
     )
 
 
-def _upsert_video_from_yt(db: Session, youtube_video_id: str) -> bool:
+def _upsert_video_from_yt(db: Session, youtube_video_id: str, polite: bool = False) -> bool:
     """Fetch fresh metadata from yt-dlp and upsert video + channel. Returns True if successful."""
-    meta = ytdlp.fetch_video_metadata(youtube_video_id)
+    meta = ytdlp.fetch_video_metadata(youtube_video_id, polite=polite)
     if not meta:
         return False
 
@@ -868,7 +868,7 @@ def get_video_by_yt_id(
         def _enrich(yt_id: str):
             bg_db = SessionLocal()
             try:
-                _upsert_video_from_yt(bg_db, yt_id)
+                _upsert_video_from_yt(bg_db, yt_id, polite=True)
             finally:
                 bg_db.close()
         background_tasks.add_task(_enrich, youtube_video_id)