feat(user): add user modification API and handler

implement user modification functionality including:
- new endpoint /userMgr/modify
- database update function DBUpdateUser
- request/response structures
- handler with proper authentication and validation
- updated API documentation
- marked as completed in README checklist

docs/api.md

@@ -277,6 +277,39 @@ X-Timestamp: 1704067200000
 
 ---
 
+## Modify User
+
+**Endpoint**: `/userMgr/modify`
+**Method**: POST
+**Content-Type**: application/json
+**Auth Required**: Yes (token)  
+**Permission Level**: visitor
+
+**Request Headers**:
+```
+X-Token: your_token
+X-Timestamp: 1704067200000
+```
+
+**Request Body**:
+```json
+{
+  "userID": 1,
+  "username": "user123",
+  "passwd": "NewPass123!"
+}
+```
+
+**Response**:
+```json
+{
+  "success": true,
+  "message": "User updated successfully"
+}
+```
+
+---
+
 ## Modify User Type
 
 **Endpoint**: `/userMgr/modifyType`