feat: add unified asset retry endpoint

backend/app/schemas/story_schemas.py

@@ -5,7 +5,6 @@ from typing import Literal
 
 from pydantic import BaseModel, Field
 
-
 MAX_DATA_LENGTH = 2000
 MAX_EDU_THEME_LENGTH = 200
 MAX_TTS_LENGTH = 4000
@@ -120,6 +119,12 @@ class StoryImageResponse(StoryStatusMixin):
     image_url: str | None
 
 
+class StoryAssetRetryRequest(BaseModel):
+    """Retry selected generated assets for a story."""
+
+    assets: list[Literal["image", "audio"]] = Field(..., min_length=1)
+
+
 class AchievementItem(BaseModel):
     """Achievement item returned for a story."""