feat: update local authentication mode to require a non-placeholder token of at least 50 characters

backend/app/core/auth_mode.py

@@ -0,0 +1,12 @@
+"""Shared auth-mode enum values."""
+
+from __future__ import annotations
+
+from enum import Enum
+
+
+class AuthMode(str, Enum):
+    """Supported authentication modes for backend and frontend."""
+
+    CLERK = "clerk"
+    LOCAL = "local"