fix: add created_at and updated_at to AdminUserResponse schema

The admin user detail page was showing empty dates because the API
response schema was missing the created_at and updated_at fields
that exist on the User model via TimestampMixin.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/admin_users.py

@@ -12,6 +12,7 @@ This module provides endpoints for:
 """
 
 import logging
+from datetime import datetime
 from typing import Optional
 
 from fastapi import APIRouter, Body, Depends, Path, Query
@@ -56,6 +57,8 @@ class AdminUserResponse(BaseModel):
     is_active: bool
     is_super_admin: bool
     platform_assignments: list[PlatformAssignmentResponse] = []
+    created_at: datetime
+    updated_at: datetime
 
     class Config:
         from_attributes = True