fix(daemons): block owner transfer while assigned

app/domain/errors.py

@@ -15,3 +15,7 @@ class DaemonNotFoundError(ValueError):
 
 class DaemonInactiveError(ValueError):
     """Raised when a new portfolio assignment targets an inactive daemon."""
+
+
+class DaemonInUseError(ValueError):
+    """Raised when an owner transfer targets a daemon assigned to a portfolio."""