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/models/channel.py

@@ -1,6 +1,6 @@
 from datetime import datetime
 
-from sqlalchemy import Boolean, DateTime, String, Text, func
+from sqlalchemy import BigInteger, Boolean, DateTime, String, Text, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.db import Base
@@ -16,6 +16,7 @@ class Channel(Base):
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
     thumbnail_url: Mapped[str | None] = mapped_column(String, nullable=True)
     uploads_playlist_id: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    subscriber_count: Mapped[int | None] = mapped_column(BigInteger, nullable=True)
     subscribed: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     last_synced_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)