Channel features: subscribers, new-videos badges, activity-driven sync

- Store subscriber count from YouTube statistics and show it on channel page
- Sync 50 videos per channel with playlistItems pagination support
- Show per-channel and per-category new-videos counters (2-day window)
- Replace hourly videos sync with activity trigger (2h idle) and
  incremental backfill (hard cap 200 per channel)
- Clicking the sidebar new-videos count filters the category feed to
  recent videos only (new_only)
- Update agent-team docs: deploy after green checks

backend/app/api/feed.py

@@ -1,10 +1,11 @@
 import base64
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy import func, or_, select
 from sqlalchemy.orm import Session
 
+from app.config import settings
 from app.core.auth_dependency import require_session
 from app.db import get_db
 from app.models.channel import Channel
@@ -80,6 +81,7 @@ def get_feed(
     channel_id: int | None = None,
     downloaded: bool = False,
     search: str | None = Query(None, max_length=200),
+    new_only: bool = False,
     limit: int = Query(DEFAULT_LIMIT, ge=1, le=MAX_LIMIT),
     cursor: str | None = None,
     db: Session = Depends(get_db),
@@ -97,6 +99,15 @@ def get_feed(
         )
         query = query.filter(Video.channel_id.in_(channel_ids_in_category))
 
+    if new_only:
+        since = datetime.now(timezone.utc) - timedelta(days=settings.new_videos_window_days)
+        query = query.filter(Video.published_at >= since)
+        # Align with the sidebar badge count (categories.py), which only
+        # counts subscribed channels: after unsubscribing, a channel's
+        # fresh videos must disappear from "new only" too.
+        subscribed_channel_ids = select(Channel.id).where(Channel.subscribed.is_(True))
+        query = query.filter(Video.channel_id.in_(subscribed_channel_ids))
+
     if downloaded:
         query = query.filter(Video.id.in_(_completed_video_ids()))