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/core/crypto.py

@@ -0,0 +1,18 @@
+from functools import lru_cache
+
+from cryptography.fernet import Fernet
+
+from app.config import settings
+
+
+@lru_cache
+def _fernet() -> Fernet:
+    return Fernet(settings.token_encryption_key.encode())
+
+
+def encrypt_token(plaintext: str) -> str:
+    return _fernet().encrypt(plaintext.encode()).decode()
+
+
+def decrypt_token(ciphertext: str) -> str:
+    return _fernet().decrypt(ciphertext.encode()).decode()