counter: fix a bug where you can't register UID card with only number inside

counter/models.py

@@ -749,7 +749,7 @@ class StudentCard(models.Model):
     @staticmethod
     def is_valid(uid):
         return (
-            uid.isupper()
+            (uid.isupper() or uid.isnumeric())
             and len(uid) == StudentCard.UID_SIZE
             and not StudentCard.objects.filter(uid=uid).exists()
         )