feat(admin): add manual reservation operations

backend/checkins/services.py

@@ -75,6 +75,29 @@ def confirm_check_in_from_token(raw_token, *, staff_user, source=CheckIn.Source.
     return CheckInResult(check_in=check_in, preview=_build_preview(reservation))
 
 
+def confirm_check_in_for_reservation(*, reservation_id, staff_user, source=CheckIn.Source.MANUAL):
+    _validate_staff_user(staff_user)
+
+    with transaction.atomic():
+        reservation = (
+            Reservation.objects.select_for_update()
+            .select_related("performance__show", "performance__venue")
+            .get(pk=reservation_id)
+        )
+        _validate_reservation_for_check_in(reservation)
+
+        try:
+            check_in = CheckIn.objects.create(
+                reservation=reservation,
+                checked_in_by=staff_user,
+                source=source,
+            )
+        except IntegrityError as exc:
+            raise AlreadyCheckedIn("Reservation has already been checked in.") from exc
+
+    return CheckInResult(check_in=check_in, preview=_build_preview(reservation))
+
+
 def _validate_staff_user(staff_user):
     if staff_user is None:
         raise MissingStaffUser("A staff user is required for check-in.")