fix: convert datetime to str in admin users list to fix JSON serializable error

app.py

@@ -3047,7 +3047,10 @@ def admin_users_list():
         cursor.execute("SELECT id, email, role, created_at FROM users ORDER BY id ASC")
         user_list = cursor.fetchall()
         for u in user_list:
-            cursor.execute("SELECT tool_path FROM user_permissions WHERE user_id = %d", (u['id'],))
+            # Convert datetime to string so Jinja2 tojson can serialize it
+            if u.get('created_at') and not isinstance(u['created_at'], str):
+                u['created_at'] = str(u['created_at'])
+            cursor.execute("SELECT tool_path FROM user_permissions WHERE user_id = %s", (u['id'],))
             u['permissions'] = [row['tool_path'] for row in cursor.fetchall()]
         conn.close()
         return render_template('tools/admin_users/index.html', users=user_list, available_tools=AVAILABLE_TOOLS)