feat(user): add createdAt field to user model

- Add createdAt field to User struct and database schema
- Update all user queries to include createdAt field
- Document new field in API documentation
- Fix server online status flag timing

docs/api.md

@@ -299,17 +299,26 @@ X-Timestamp: 1704067200000
     {
       "userID": 1,
       "username": "admin",
-      "type": "superuser"
+      "type": "superuser",
+      "createdAt": "2024-01-01 12:00:00"
     },
     {
       "userID": 2,
       "username": "new_user",
-      "type": "admin"
+      "type": "admin",
+      "createdAt": "2024-01-02 14:30:00"
     }
   ]
 }
 ```
 
+| Field | Type | Description |
+|-------|------|-------------|
+| userID | int | User ID |
+| username | string | Username |
+| type | string | User type: superuser, admin, visitor |
+| createdAt | string | User creation time (YYYY-MM-DD HH:MM:SS format) |
+
 ---
 
 ## List Active Sessions