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

database.go

@@ -346,6 +346,19 @@ func DBQuerySpecificUser(userID int) (User, error) { // Query user by ID
 	return user, nil
 }
 
+func DBUpdateUser(userID int, username, passwd string) error {
+	var err error
+	if passwd == "" {
+		_, err = db.Exec("UPDATE userLogin SET username = ? WHERE userID = ?", username, userID)
+	} else {
+		_, err = db.Exec("UPDATE userLogin SET username = ?, passwd = ? WHERE userID = ?", username, passwd, userID)
+	}
+	if err != nil {
+		return fmt.Errorf("failed to update user: %w", err)
+	}
+	return nil
+}
+
 func DBUpdateUserType (userID int, newType string) error {
 	_, err := db.Exec("UPDATE userLogin SET type = ? WHERE userID = ?", newType, userID)
 	if err != nil {