feat: add flow export/import backend (migration, endpoints, schemas)

Add .rfflow file export/import support:
- Migration 050: import_metadata JSONB column on trees
- GET /trees/{id}/export?format=json|xml endpoint
- POST /trees/import endpoint (creates draft, resolves categories/tags)
- FlowExportEnvelope, FlowImportRequest/Response schemas
- import_metadata field on TreeResponse

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

backend/app/api/endpoints/trees.py

@@ -116,7 +116,8 @@ def build_full_tree_response(tree: Tree, parent_tree: Tree = None) -> TreeRespon
         version=tree.version,
         usage_count=tree.usage_count,
         created_at=tree.created_at,
-        updated_at=tree.updated_at
+        updated_at=tree.updated_at,
+        import_metadata=tree.import_metadata
     )