feat: improve admin and add demo data command

backend/shows/test_admin.py

@@ -0,0 +1,13 @@
+from django.contrib import admin
+from django.test import SimpleTestCase
+
+from bookings.models import Reservation, ReservationToken
+from checkins.models import CheckIn
+from shows.models import Performance, Show, Venue
+
+
+class AdminRegistrationTests(SimpleTestCase):
+    def test_core_models_are_registered_in_admin(self):
+        for model in (Show, Venue, Performance, Reservation, ReservationToken, CheckIn):
+            with self.subTest(model=model.__name__):
+                self.assertTrue(admin.site.is_registered(model))