feat: add graph workflow fallback and retry metadata

backend/app/domains/rendering/workflow_config_utils.py

@@ -17,6 +17,8 @@ _PRESET_TYPES = {
     "custom",
 }
 
+_EXECUTION_MODES = {"legacy", "graph", "shadow"}
+
 _NODE_TYPE_TO_STEP: dict[str, str] = {
     "inputNode": StepName.RESOLVE_STEP_PATH.value,
     "convertNode": StepName.STL_CACHE_GENERATE.value,
@@ -243,6 +245,15 @@ def get_workflow_preset_type(config: dict[str, Any]) -> str | None:
     return None
 
 
+def get_workflow_execution_mode(config: dict[str, Any], *, default: str = "legacy") -> str:
+    canonical = canonicalize_workflow_config(config)
+    ui = canonical.get("ui") or {}
+    mode = ui.get("execution_mode")
+    if mode in _EXECUTION_MODES:
+        return mode
+    return default
+
+
 def extract_runtime_workflow(config: dict[str, Any]) -> tuple[str | None, dict[str, Any]]:
     canonical = canonicalize_workflow_config(config)
     preset = get_workflow_preset_type(canonical)