feat: M2 (part 1) — subscription import, YouTube client, quota guard

- Channel/Subscription/Video/ApiQuotaUsage models + migration 0002
- Synchronous YouTube Data API client with OAuth token refresh and per-call
  quota accounting (API key preferred for public reads when configured)
- Subscription import: upsert channels + subscription links, prune unsubscribed,
  fetch channel details (uploads playlist, topics, stats, handle, country)
- Central quota guard tracking units per US-Pacific day against a daily budget
- POST /api/sync/subscriptions and GET /api/sync/status endpoints

backend/app/main.py

@@ -8,7 +8,7 @@ from starlette.middleware.sessions import SessionMiddleware
 
 from app import auth
 from app.config import settings
-from app.routes import health
+from app.routes import health, sync
 
 app = FastAPI(title=settings.app_name)
 
@@ -30,6 +30,7 @@ if settings.frontend_origin:
 
 app.include_router(health.router)
 app.include_router(auth.router)
+app.include_router(sync.router)
 
 STATIC_DIR = Path(__file__).parent / "static"
 app.mount("/assets", StaticFiles(directory=STATIC_DIR / "assets"), name="assets")