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

migrations/versions/0008_channel_subscriber_count.py

@@ -0,0 +1,26 @@
+"""channels.subscriber_count
+
+Revision ID: 0008_channel_subscriber_count
+Revises: 0007_drop_access_expiry
+Create Date: 2026-09-17
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0008_channel_subscriber_count"
+down_revision: Union[str, None] = "0007_drop_access_expiry"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Subscriber count comes from YouTube channels.list (statistics part);
+    # hidden counts stay NULL.
+    op.add_column("channels", sa.Column("subscriber_count", sa.BigInteger(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("channels", "subscriber_count")