Add min/max-limit on giftcard_expiry_years

src/pretix/base/settings.py

@@ -3291,6 +3291,8 @@ Your {organizer} team"""))  # noqa: W291
             label=_('Validity of gift card codes in years'),
             help_text=_('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,
+            max_value=99,
         )
     },
     'cookie_consent': {