Add validation to prevent negative values in gift card settings

app/eventyay/base/configurations/default_setting.py

@@ -2422,6 +2422,7 @@ def primary_font_kwargs():
                 'The system generates by default {}-character long gift card codes. However, if a different length '
                 'is required, it can be set here.'.format(settings.ENTROPY['giftcard_secret'])
             ),
+            min_value=1,
         ),
     },
     'giftcard_expiry_years': {
@@ -2435,6 +2436,7 @@ def primary_font_kwargs():
                 'If you set a number here, gift cards will by default expire at the end of the year after this '
                 'many years. If you keep it empty, gift cards do not have an explicit expiry date.'
             ),
+            min_value=0,
         ),
     },
     'privacy_policy': {