Implement Phases 1-5: skeleton, OAuth, categories, video sync/feed, playback

- FastAPI + PostgreSQL + Alembic + React/Vite skeleton, Docker Compose, healthcheck
- Google OAuth (single allowed account), encrypted refresh token storage
- Subscriptions sync with pagination, uploads playlist batch fetch
- Categories CRUD, many-to-many channel assignment, category filtering
- Video sync (playlistItems + videos.list batching), cached feed with cursor
  pagination, background scheduler (APScheduler)
- Video detail page with YouTube embed player
- SPA fallback routing, optimistic UI updates, client-side query caching

40 backend tests covering OAuth allow-list, sync idempotency, cascade deletes,
cursor pagination, and category filtering.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/services/scheduler.py

@@ -0,0 +1,50 @@
+import logging
+
+from apscheduler.schedulers.background import BackgroundScheduler
+
+from app.config import settings
+from app.db import SessionLocal
+from app.services import sync
+
+logger = logging.getLogger(__name__)
+
+
+def _run_subscriptions_sync() -> None:
+    db = SessionLocal()
+    try:
+        sync.sync_subscriptions(db)
+    except sync.SyncInProgress:
+        logger.info("Scheduled subscriptions sync skipped: already running")
+    except Exception:
+        logger.exception("Scheduled subscriptions sync failed")
+    finally:
+        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:
+    scheduler = BackgroundScheduler(timezone="UTC")
+    scheduler.add_job(
+        _run_subscriptions_sync,
+        "interval",
+        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