feat: Add created_by and cost fields to generation models, populate created_by from the authenticated user, and implement cost calculation.

api/models/GenerationRequest.py

@@ -42,6 +42,8 @@ class GenerationResponse(BaseModel):
     input_token_usage: Optional[int] = None
     output_token_usage: Optional[int] = None
     progress: int = 0
+    cost: Optional[float] = None
+    created_by: Optional[str] = None
     created_at: datetime = datetime.now(UTC)
     updated_at: datetime = datetime.now(UTC)