feat: add ax serve command with JSON API backed by NodeService

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

service/node_service.go

@@ -82,3 +82,14 @@ func GetNodeService(cfg Config) (NodeService, error) {
 	}
 	return &nodeServiceImpl{store: st, userID: user}, nil
 }
+
+func GetNodeServiceForUser(user string) (NodeService, error) {
+	if user == "" {
+		return nil, fmt.Errorf("user is required")
+	}
+	st, err := store.FindAndOpenSQLiteStore()
+	if err != nil {
+		return nil, err
+	}
+	return &nodeServiceImpl{store: st, userID: user}, nil
+}