fix: stop discovery from bursting dozens of yt-dlp calls inside one task

Each search/graph/trending task was calling _fetch_and_index_channel
inline for up to 10-15 newly discovered channels, each making up to 4
yt-dlp calls (1 channel metadata + 3 individual video fetches for
dateless entries). This bypassed the 30-90 s worker gap, producing
bursts of 40-60 calls in rapid succession and hammering YouTube.

Changes:
- _fetch_and_index_channel: removed the dateless-video individual
  fetch loop — one call per channel, videos without published_at are
  simply skipped at discovery time
- _search_and_store and _fetch_graph_for_channel: queue channel
  indexing as separate worker tasks (3 and 2 respectively) so the
  30-90 s gap applies between every yt-dlp call, including channel
  indexing
- update_trending_signal and update_graph_signal (old sync path):
  removed inline _fetch_and_index_channel loops (15 and 10 channels)
- _discovery_task in channels.py: replaced run_full_discovery (old
  synchronous path) with schedule_discovery so sync-all and
  follow-by-url go through the queue system

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

backend/routers/channels.py

@@ -173,15 +173,11 @@ def _index_channel_task(channel_id: int, user_id: int, max_videos: int = 30):
 
 
 def _discovery_task(user_id: int):
-    from ..database import SessionLocal
-    from ..services.discovery import run_full_discovery
-    db = SessionLocal()
+    from ..services.discovery import schedule_discovery
     try:
-        run_full_discovery(db, user_id)
+        schedule_discovery(user_id)
     except Exception:
         pass
-    finally:
-        db.close()
 
 
 def _enrich_missing_task(limit: int = 20):