feat: add Django backend skeleton

backend/checkins/__init__.py

@@ -0,0 +1 @@
+

backend/checkins/apps.py

@@ -0,0 +1,6 @@
+from django.apps import AppConfig
+
+
+class CheckinsConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "checkins"

backend/checkins/models.py

@@ -0,0 +1 @@
+# Domain models will be added when check-in behavior is implemented.