fix: Workflow executor graph parsing and error handling

Fixes:
- graph_data was incorrectly json.dumps() encoded (should stay as dict)
- workflow_id=None in error handler caused ValidationError
- parse_workflow_graph expects Dict, not str

Changes:
- Use graph_dict directly instead of json.dumps(graph_data)
- Set workflow_id="" when None in error handler

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/workflow_executor.py

@@ -79,8 +79,8 @@ async def execute_workflow(
     try:
         # 1. Lade Workflow-Definition
         if graph_data:
-            # Phase 5: Graph direkt aus ai_prompts.graph_data
-            graph_json = json.dumps(graph_data)
+            # Phase 5: Graph direkt aus ai_prompts.graph_data (already a dict)
+            graph_dict = graph_data
             logger.debug(f"Using provided graph_data")
         elif workflow_id:
             # Phase 0-4: Graph aus workflow_definitions Tabelle (legacy)
@@ -94,13 +94,13 @@ async def execute_workflow(
                 if not row:
                     raise ValueError(f"Workflow not found: {workflow_id}")
 
-                graph_json = row['graph']
+                graph_dict = row['graph']  # PostgreSQL JSONB returns dict
             logger.debug(f"Loaded graph from workflow_definitions: {workflow_id}")
         else:
             raise ValueError("Entweder workflow_id oder graph_data muss übergeben werden")
 
         # 2. Parse Graph
-        graph = parse_workflow_graph(graph_json)
+        graph = parse_workflow_graph(graph_dict)
         logger.debug(f"Parsed graph: {len(graph.nodes)} nodes, {len(graph.edges)} edges")
 
         # 4. Lade Question Catalog
@@ -201,7 +201,7 @@ async def execute_workflow(
         completed_at = datetime.utcnow().isoformat()
         save_execution_state(
             execution_id=execution_id,
-            workflow_id=workflow_id,
+            workflow_id=workflow_id or "",  # Empty string if None (when graph_data is used)
             profile_id=profile_id,
             node_states=node_states if 'node_states' in locals() else [],
             status="failed",
@@ -212,7 +212,7 @@ async def execute_workflow(
 
         return ExecutionResult(
             execution_id=execution_id,
-            workflow_id=workflow_id,
+            workflow_id=workflow_id or "",  # Empty string if None (when graph_data is used)
             status="failed",
             node_states=node_states if 'node_states' in locals() else [],
             aggregated_result={},