hscontrol: use Updates() instead of Save() for partial updates

Changed UpdateUser and re-registration flows to use Updates() which only
writes modified fields, preventing unintended overwrites of unchanged fields.

Also updated UsePreAuthKey to use Model().Update() for single field updates
and removed unused NodeSave wrapper.

hscontrol/db/preauth_keys.go

@@ -145,11 +145,12 @@ func (hsdb *HSDatabase) ExpirePreAuthKey(k *types.PreAuthKey) error {
 
 // UsePreAuthKey marks a PreAuthKey as used.
 func UsePreAuthKey(tx *gorm.DB, k *types.PreAuthKey) error {
-	k.Used = true
-	if err := tx.Save(k).Error; err != nil {
+	err := tx.Model(k).Update("used", true).Error
+	if err != nil {
 		return fmt.Errorf("failed to update key used status in the database: %w", err)
 	}
 
+	k.Used = true
 	return nil
 }