feat: add graph workflow fallback and retry metadata

backend/app/domains/rendering/workflow_schema.py

@@ -16,6 +16,8 @@ Example config::
         ]
     }
 """
+from typing import Literal
+
 from pydantic import BaseModel, Field, field_validator, model_validator
 
 from app.core.process_steps import StepName
@@ -49,6 +51,7 @@ class WorkflowEdge(BaseModel):
 
 class WorkflowUI(BaseModel):
     preset: str | None = None
+    execution_mode: Literal["legacy", "graph", "shadow"] | None = None
 
 
 class WorkflowConfig(BaseModel):