feat: move unified generation to background worker

backend/app/api/stories.py

@@ -67,13 +67,13 @@ def _mark_legacy_generation_endpoint(response: Response, successor: str) -> None
     response.headers.update(_legacy_generation_headers(successor))
 
 
-@router.post("/generations", response_model=GenerationResponse)
+@router.post("/generations", response_model=GenerationResponse, status_code=202)
 async def create_generation(
     request: GenerationRequest,
     user: User = Depends(require_user),
     db: AsyncSession = Depends(get_db),
 ):
-    """Create a story or storybook through the unified generation workflow."""
+    """Accept one story/storybook generation request for background execution."""
     await check_rate_limit(f"story:{user.id}", RATE_LIMIT_REQUESTS, RATE_LIMIT_WINDOW)
     return await story_service.generate_generation_service(request, user.id, db)