refactor: align backend layout and supabase infra

Consolidate backend modules/tests under the backend package while syncing Supabase compose/env config and related plans.

backend/src/core/auth/models.py

@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from uuid import UUID
+
+
+@dataclass(frozen=True)
+class CurrentUser:
+    id: UUID