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

tests/test_channels.py

@@ -1,3 +1,5 @@
+from datetime import datetime, timedelta, timezone
+
 import pytest
 from fastapi.testclient import TestClient
 
@@ -5,6 +7,7 @@ from app.core.auth_dependency import require_session
 from app.db import get_db
 from app.main import app
 from app.models.channel import Channel
+from app.models.video import Video
 from app.services import sync
 from app.services.youtube_client import YouTubeInsufficientScope
 
@@ -86,3 +89,54 @@ def test_unsubscribe_insufficient_scope_returns_403(client, db_session, monkeypa
 def test_unsubscribe_channel_not_found(client):
     resp = client.post("/api/channels/9999/unsubscribe")
     assert resp.status_code == 404
+
+
+def _seed_video(db_session, channel, video_id, published_at):
+    video = Video(
+        youtube_video_id=video_id,
+        channel_id=channel.id,
+        title=f"Video {video_id}",
+        published_at=published_at,
+        youtube_url=f"https://www.youtube.com/watch?v={video_id}",
+    )
+    db_session.add(video)
+    db_session.commit()
+    return video
+
+
+def test_channel_responses_include_subscriber_and_new_videos_counts(client, db_session):
+    channel = _seed_channel(db_session)
+    channel.subscriber_count = 1200000
+    db_session.commit()
+
+    now = datetime.now(timezone.utc)
+    _seed_video(db_session, channel, "vidRecent", now - timedelta(days=1))
+    _seed_video(db_session, channel, "vidOld", now - timedelta(days=30))
+
+    resp = client.get("/api/channels")
+    assert resp.status_code == 200
+    payload = resp.json()
+    assert len(payload) == 1
+    assert payload[0]["subscriber_count"] == 1200000
+    # Only the video from 1 day ago is inside the 7-day window.
+    assert payload[0]["new_videos_count"] == 1
+
+    resp_single = client.get(f"/api/channels/{channel.id}")
+    assert resp_single.status_code == 200
+    single = resp_single.json()
+    assert single["subscriber_count"] == 1200000
+    assert single["new_videos_count"] == 1
+
+
+def test_channel_new_videos_count_zero_without_recent_videos(client, db_session):
+    channel = _seed_channel(db_session)
+
+    now = datetime.now(timezone.utc)
+    _seed_video(db_session, channel, "vidOld", now - timedelta(days=30))
+
+    resp = client.get("/api/channels")
+    assert resp.status_code == 200
+    payload = resp.json()
+    assert len(payload) == 1
+    assert payload[0]["subscriber_count"] is None
+    assert payload[0]["new_videos_count"] == 0