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/state.py

@@ -0,0 +1,18 @@
+from sqlalchemy.orm import Session
+
+from app.models.app_settings import AppSetting
+
+
+def get_setting(db: Session, key: str) -> str | None:
+    row = db.query(AppSetting).filter(AppSetting.key == key).one_or_none()
+    return row.value if row else None
+
+
+def set_setting(db: Session, key: str, value: str) -> None:
+    row = db.query(AppSetting).filter(AppSetting.key == key).one_or_none()
+    if row is None:
+        row = AppSetting(key=key, value=value)
+        db.add(row)
+    else:
+        row.value = value
+    db.commit()