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/config.py

@@ -0,0 +1,32 @@
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
+
+    app_env: str = "production"
+    app_base_url: str = "http://localhost:8080"
+    app_port: int = 8080
+    app_secret_key: str
+    token_encryption_key: str
+
+    database_url: str
+
+    google_client_id: str = ""
+    google_client_secret: str = ""
+    google_redirect_uri: str = ""
+    allowed_google_email: str = ""
+
+    metube_api_base_url: str = "http://127.0.0.1:8081"
+    metube_public_base_url: str = "http://127.0.0.1:8081"
+    metube_container_download_dir: str = "/downloads"
+    metube_request_timeout_seconds: int = 30
+
+    subscriptions_sync_interval_hours: int = 6
+    videos_sync_interval_minutes: int = 60
+    videos_per_channel_sync: int = 10
+
+    log_level: str = "INFO"
+
+
+settings = Settings()