refactor: consolidate user and superadmin sessions into unified model

gatehouse_app/utils/constants.py

@@ -52,6 +52,13 @@ class SessionStatus(str, Enum):
     REVOKED = "revoked"
 
 
+class SessionType(str, Enum):
+    """Session owner type discriminator."""
+
+    USER = "user"
+    SUPERADMIN = "superadmin"
+
+
 class AuditAction(str, Enum):
     """Audit log action types."""