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/services/discovery.py

@@ -23,7 +23,7 @@ _worker_lock = _threading.Lock()
 
 
 def _fetch_and_index_channel(db: Session, channel: Channel):
-    """Fetch full metadata + recent videos for a discovered channel."""
+    """Fetch metadata + recent videos for a discovered channel (one yt-dlp call only)."""
     try:
         result = ytdlp.fetch_channel_metadata(channel.youtube_channel_id, max_videos=10, polite=True)
         if not result:
@@ -34,32 +34,9 @@ def _fetch_and_index_channel(db: Session, channel: Channel):
                 setattr(channel, k, v)
         channel.crawled_at = datetime.utcnow()
 
-        videos = result.get("videos", [])
-
-        # For videos missing a date (RSS didn't cover them or flat-playlist had no timestamp),
-        # do individual fetches — capped at 3 to avoid slow-downs.
-        dateless = [v for v in videos if not v.get("published_at")]
-        individual_fetched: dict[str, dict] = {}
-        for vdata in dateless[:3]:
+        for vdata in result.get("videos", []):
             yt_id = vdata.get("youtube_video_id")
-            if not yt_id:
-                continue
-            try:
-                meta = ytdlp.fetch_video_metadata(yt_id, polite=True)
-                if meta and meta.get("published_at"):
-                    individual_fetched[yt_id] = meta
-            except Exception:
-                pass
-
-        for vdata in videos:
-            yt_id = vdata.get("youtube_video_id")
-            if not yt_id:
-                continue
-            # Prefer individually-fetched metadata if we retrieved it
-            if yt_id in individual_fetched:
-                vdata = individual_fetched[yt_id]
-            # Skip videos we still can't date — undated videos break feed ordering
-            if not vdata.get("published_at"):
+            if not yt_id or not vdata.get("published_at"):
                 continue
             if not db.query(Video).filter_by(youtube_video_id=yt_id).first():
                 db.add(Video(
@@ -194,10 +171,9 @@ def _search_and_store(
 
     db.commit()
 
-    for channel_id in needs_indexing[:10]:
-        channel = db.query(Channel).filter_by(id=channel_id).first()
-        if channel:
-            _fetch_and_index_channel(db, channel)
+    # Queue channel indexing as separate worker tasks (30-90 s gaps apply).
+    for channel_id in needs_indexing[:3]:
+        _task_queue.put((user_id, lambda cid=channel_id: _do_task_index_channel(user_id, cid)))
 
 
 def crawl_by_search(db: Session, user_id: int):
@@ -599,11 +575,6 @@ def update_trending_signal(db: Session, user_id: int, regions: list[str]):
 
     db.commit()
 
-    for channel_id in needs_indexing[:10]:
-        channel = db.query(Channel).filter_by(id=channel_id).first()
-        if channel:
-            _fetch_and_index_channel(db, channel)
-
 
 def update_graph_signal(db: Session, user_id: int):
     """Discover channels featured on followed channels' /channels tab.
@@ -664,11 +635,6 @@ def update_graph_signal(db: Session, user_id: int):
 
     db.commit()
 
-    for channel_id in needs_indexing[:15]:
-        channel = db.query(Channel).filter_by(id=channel_id).first()
-        if channel:
-            _fetch_and_index_channel(db, channel)
-
 
 def run_full_discovery(db: Session, user_id: int, regions: list[str] | None = None):
     if regions is None:
@@ -788,10 +754,8 @@ def _fetch_graph_for_channel(db: Session, user_id: int, source_yt_id: str):
 
     db.commit()
 
-    for channel_id in needs_indexing[:3]:
-        ch = db.query(Channel).filter_by(id=channel_id).first()
-        if ch:
-            _fetch_and_index_channel(db, ch)
+    for channel_id in needs_indexing[:2]:
+        _task_queue.put((user_id, lambda cid=channel_id: _do_task_index_channel(user_id, cid)))
 
 
 def _do_task_graph(user_id: int, source_yt_id: str):
@@ -803,6 +767,19 @@ def _do_task_graph(user_id: int, source_yt_id: str):
         db.close()
 
 
+def _do_task_index_channel(user_id: int, channel_id: int):
+    """Index one newly-discovered channel (one yt-dlp call). Queued as a separate
+    worker task so the 30-90 s gap applies rather than bursting inline."""
+    from ..database import SessionLocal
+    db = SessionLocal()
+    try:
+        channel = db.query(Channel).filter_by(id=channel_id).first()
+        if channel:
+            _fetch_and_index_channel(db, channel)
+    finally:
+        db.close()
+
+
 def _worker_loop():
     while True:
         try: