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/services/scheduler.py

@@ -21,19 +21,9 @@ def _run_subscriptions_sync() -> None:
         db.close()
 
 
-def _run_videos_sync() -> None:
-    db = SessionLocal()
-    try:
-        sync.sync_videos(db)
-    except sync.SyncInProgress:
-        logger.info("Scheduled videos sync skipped: already running")
-    except Exception:
-        logger.exception("Scheduled videos sync failed")
-    finally:
-        db.close()
-
-
 def create_scheduler() -> BackgroundScheduler:
+    # Videos sync is not scheduled anymore: it runs on user activity via
+    # services.sync_trigger.maybe_trigger_videos_sync (see require_session).
     scheduler = BackgroundScheduler(timezone="UTC")
     scheduler.add_job(
         _run_subscriptions_sync,
@@ -41,10 +31,4 @@ def create_scheduler() -> BackgroundScheduler:
         hours=settings.subscriptions_sync_interval_hours,
         id="subscriptions_sync",
     )
-    scheduler.add_job(
-        _run_videos_sync,
-        "interval",
-        minutes=settings.videos_sync_interval_minutes,
-        id="videos_sync",
-    )
     return scheduler