Auto-reindex channel on page visit if stale

GET /channels/{id} now fires a background _index_channel_task if the
channel hasn't been crawled in the last hour. The frontend refetches
channel + videos 8s after page load to pick up the updated data.

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

backend/routers/channels.py

@@ -546,6 +546,7 @@ def bulk_channel_action(
 @router.get("/{channel_id}", response_model=ChannelOut)
 def get_channel(
     channel_id: int,
+    background_tasks: BackgroundTasks,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
@@ -576,6 +577,16 @@ def get_channel(
     ).mappings().first()
     if not row:
         raise HTTPException(status_code=404, detail="Channel not found")
+
+    # Re-index in the background if stale (not crawled in the last hour)
+    crawled_at = row.get("crawled_at")
+    stale = (
+        crawled_at is None
+        or (datetime.utcnow() - crawled_at).total_seconds() > 3600
+    )
+    if stale:
+        background_tasks.add_task(_index_channel_task, channel_id, current_user.id)
+
     return ChannelOut(**dict(row))