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/sync_trigger.py

@@ -0,0 +1,91 @@
+"""Activity-triggered videos sync.
+
+The hourly scheduled videos sync is gone; instead, every authenticated request
+(require_session) calls maybe_trigger_videos_sync(). The check must stay cheap:
+if the sync is not running and enough idle time has passed since the last
+completed sync, a background thread is started which calls sync.sync_videos
+with its own DB session.
+"""
+
+import json
+import logging
+import threading
+from datetime import datetime, timedelta, timezone
+
+from app.config import settings
+from app.db import SessionLocal
+from app.services import sync
+from app.services.state import get_setting
+
+logger = logging.getLogger(__name__)
+
+
+def is_videos_sync_due(finished_at_iso: str | None, now: datetime, idle_hours: int) -> bool:
+    """Pure predicate: should an automatic videos sync start now?
+
+    True when the last completed sync's finished_at is missing/unparseable or
+    older than `idle_hours`."""
+    if not finished_at_iso:
+        return True
+    if not isinstance(finished_at_iso, str):
+        return True
+    try:
+        finished_at = datetime.fromisoformat(finished_at_iso.replace("Z", "+00:00"))
+    except (ValueError, TypeError):
+        return True
+    if finished_at.tzinfo is None:
+        finished_at = finished_at.replace(tzinfo=timezone.utc)
+    if now.tzinfo is None:
+        now = now.replace(tzinfo=timezone.utc)
+    return now - finished_at >= timedelta(hours=idle_hours)
+
+
+def _extract_finished_at(raw: str | None) -> str | None:
+    if not raw:
+        return None
+    try:
+        payload = json.loads(raw)
+    except (ValueError, TypeError):
+        return None
+    if not isinstance(payload, dict):
+        return None
+    return payload.get("finished_at")
+
+
+def _run_videos_sync() -> None:
+    db = SessionLocal()
+    try:
+        sync.sync_videos(db)
+    except sync.SyncInProgress:
+        # Lost the race against a manual sync or another trigger: fine, the
+        # other run will record the status.
+        logger.info("Triggered videos sync skipped: already running")
+    except Exception:
+        logger.exception("Triggered videos sync failed")
+    finally:
+        db.close()
+
+
+def maybe_trigger_videos_sync() -> None:
+    """Cheap per-request check; never raises. Starts a background videos sync
+    when due, or does nothing when a sync is already running."""
+    try:
+        if sync.is_videos_sync_running():
+            return
+
+        db = SessionLocal()
+        try:
+            raw = get_setting(db, sync.VIDEOS_SYNC_STATUS_KEY)
+            finished_at = _extract_finished_at(raw)
+            if not is_videos_sync_due(finished_at, datetime.now(timezone.utc), settings.videos_sync_idle_hours):
+                return
+        finally:
+            db.close()
+
+        logger.info(
+            "Videos sync due (last finished: %s), starting in background thread", finished_at
+        )
+        threading.Thread(target=_run_videos_sync, daemon=True).start()
+    except Exception:
+        # Never break the request because of the trigger itself.
+        logger.exception("Failed to evaluate videos sync trigger")