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/models/__init__.py

@@ -0,0 +1,8 @@
+from app.models.app_settings import AppSetting
+from app.models.category import Category
+from app.models.channel import Channel
+from app.models.channel_category import channel_categories
+from app.models.oauth_credentials import OAuthCredentials
+from app.models.video import Video
+
+__all__ = ["AppSetting", "Category", "Channel", "channel_categories", "OAuthCredentials", "Video"]