dont overwrite other bucket shifts during assignment

shiftregister/fallback/models.py

@@ -51,7 +51,9 @@ class TeamMember(models.Model):
         )
         # easy part: enough free shifts for everyone:
         shifts = free_bucket.order_by("?")[:shift_count]
-        self.fallback_shifts.set(shifts)
+        for shift in shifts:
+            self.fallback_shifts.add(shift)
+
         # there is a chance that even if qota*teammembers team members are activatet, there are still unasigned shifts
         # this happens if there are shifts with multiple people left, as we can not assign multiple slots for
         # the same shift to one member.