fix: Make message sender field more flexible to fix 422 error

modules/text_story/schemas.py

@@ -9,7 +9,7 @@ from typing import List, Optional, Literal
 
 class Message(BaseModel):
     """Single chat message."""
-    sender: Literal["A", "B"] = Field(
+    sender: str = Field(
         description="A = User (right, blue bubble), B = Other person (left, gray bubble)"
     )
     text: str = Field(