Reduce parallel YouTube request workers to avoid cookie invalidation

8 simultaneous yt-dlp processes hitting video pages looks like a bot
attack and causes YouTube to nuke the session cookies. Drop to:
- Popular fetch view_count enrichment: 8→3 workers
- Discovery search: 8→4 workers
- Graph signal (featured channels): 8→3 workers

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

backend/routers/channels.py

@@ -742,7 +742,7 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str):
     if not video_ids:
         return
 
-    with ThreadPoolExecutor(max_workers=8) as pool:
+    with ThreadPoolExecutor(max_workers=3) as pool:
         futures = {pool.submit(ytdlp.fetch_video_metadata, vid): vid for vid in video_ids}
         results = {}
         for future in as_completed(futures):