fix: Add ON CONFLICT to workflow_executions insert

Prevents duplicate key violation when save_execution_state is called
multiple times with the same execution_id (e.g., during error handling).

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

backend/workflow_executor.py

@@ -880,6 +880,11 @@ def save_execution_state(
             INSERT INTO workflow_executions
             (id, workflow_id, profile_id, status, node_states, execution_log, started_at, completed_at)
             VALUES (%s, %s, %s, %s, %s, %s, %s, %s)
+            ON CONFLICT (id) DO UPDATE SET
+                status = EXCLUDED.status,
+                node_states = EXCLUDED.node_states,
+                execution_log = EXCLUDED.execution_log,
+                completed_at = EXCLUDED.completed_at
         """, (
             execution_id,
             workflow_id,