fix: pass explicit status='draft' when creating AI-generated flow

Tree model defaults to 'published' in the DB schema, but passing status=None
from the constructor overrides that default, causing a nullable=False violation
and a 500 on save.

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

frontend/src/components/ai-builder/AIFlowBuilderModal.tsx

@@ -56,6 +56,7 @@ export function AIFlowBuilderModal({ isOpen, onClose }: AIFlowBuilderModalProps)
         description: assembledTree.suggested_description,
         tree_structure: assembledTree.tree_structure,
         tree_type: metadata.flow_type,
+        status: 'draft',
       })
       handleClose()
       const editorPath =