feat(user): add user type modification endpoint

- Implement new endpoint `/userMgr/modifyType` for updating user types
- Add database function DBUpdateUserType to handle type updates
- Include request validation and proper error handling
- Update API documentation with new endpoint details

docs/api.md

@@ -277,6 +277,38 @@ X-Timestamp: 1704067200000
 
 ---
 
+## Modify User Type
+
+**Endpoint**: `/userMgr/modifyType`  
+**Method**: POST  
+**Content-Type**: application/json  
+**Auth Required**: Yes (token)  
+**Permission Level**: Superuser
+
+**Request Headers**:
+```
+X-Token: your_token
+X-Timestamp: 1704067200000
+```
+
+**Request Body**:
+```json
+{
+  "userID": 2,
+  "newType": "superadmin"
+}
+```
+
+**Response**:
+```json
+{
+  "success": true,
+  "message": "User type updated successfully"
+}
+```
+
+---
+
 ## List Users
 
 **Endpoint**: `/userMgr/list`