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/channels.py

@@ -1,15 +1,18 @@
 import logging
+from datetime import datetime, timedelta, timezone
 
 from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
-from sqlalchemy import select
+from sqlalchemy import func, 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.category import Category
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
+from app.models.video import Video
 from app.services import sync
 from app.services.google_oauth import OAuthNotConnected
 from app.services.youtube_client import YouTubeAPIError, YouTubeInsufficientScope, YouTubeQuotaExceeded
@@ -37,7 +40,22 @@ def _category_ids_by_channel(db: Session, channel_ids: list[int]) -> dict[int, l
     return result
 
 
-def _serialize(channel: Channel, category_ids: list[int]) -> dict:
+def _new_videos_counts(db: Session, channel_ids: list[int]) -> dict[int, int]:
+    """Videos published within the new-videos window, per channel, in one
+    aggregate query."""
+    if not channel_ids:
+        return {}
+    since = datetime.now(timezone.utc) - timedelta(days=settings.new_videos_window_days)
+    rows = (
+        db.query(Video.channel_id, func.count(Video.id))
+        .filter(Video.channel_id.in_(channel_ids), Video.published_at >= since)
+        .group_by(Video.channel_id)
+        .all()
+    )
+    return {channel_id: count for channel_id, count in rows}
+
+
+def _serialize(channel: Channel, category_ids: list[int], new_videos_count: int = 0) -> dict:
     return {
         "id": channel.id,
         "youtube_channel_id": channel.youtube_channel_id,
@@ -45,9 +63,11 @@ def _serialize(channel: Channel, category_ids: list[int]) -> dict:
         "description": channel.description,
         "thumbnail_url": channel.thumbnail_url,
         "uploads_playlist_id": channel.uploads_playlist_id,
+        "subscriber_count": channel.subscriber_count,
         "subscribed": channel.subscribed,
         "last_synced_at": channel.last_synced_at,
         "category_ids": category_ids,
+        "new_videos_count": new_videos_count,
     }
 
 
@@ -75,8 +95,10 @@ def list_channels(
         query = query.filter(Channel.id.in_(channel_ids_in_category))
 
     channels = query.order_by(Channel.title.asc()).all()
-    category_map = _category_ids_by_channel(db, [c.id for c in channels])
-    return [_serialize(c, category_map.get(c.id, [])) for c in channels]
+    channel_ids = [c.id for c in channels]
+    category_map = _category_ids_by_channel(db, channel_ids)
+    new_videos_map = _new_videos_counts(db, channel_ids)
+    return [_serialize(c, category_map.get(c.id, []), new_videos_map.get(c.id, 0)) for c in channels]
 
 
 @router.get("/channels/{channel_id}")
@@ -85,7 +107,8 @@ def get_channel(channel_id: int, db: Session = Depends(get_db)) -> dict:
     if channel is None:
         raise HTTPException(status_code=404, detail="Channel not found")
     category_map = _category_ids_by_channel(db, [channel_id])
-    return _serialize(channel, category_map.get(channel_id, []))
+    new_videos_map = _new_videos_counts(db, [channel_id])
+    return _serialize(channel, category_map.get(channel_id, []), new_videos_map.get(channel_id, 0))
 
 
 @router.put("/channels/{channel_id}/categories")
@@ -110,7 +133,8 @@ def set_channel_categories(channel_id: int, payload: ChannelCategoriesUpdate, db
         )
     db.commit()
 
-    return _serialize(channel, sorted(unique_ids))
+    new_videos_map = _new_videos_counts(db, [channel_id])
+    return _serialize(channel, sorted(unique_ids), new_videos_map.get(channel_id, 0))
 
 
 @router.post("/channels/{channel_id}/unsubscribe")
@@ -139,4 +163,5 @@ def unsubscribe_channel(channel_id: int, db: Session = Depends(get_db)) -> dict:
         raise HTTPException(status_code=502, detail="YouTube is unavailable")
 
     category_map = _category_ids_by_channel(db, [channel_id])
-    return _serialize(channel, category_map.get(channel_id, []))
+    new_videos_map = _new_videos_counts(db, [channel_id])
+    return _serialize(channel, category_map.get(channel_id, []), new_videos_map.get(channel_id, 0))