Add shop, POS sync, event ticketing, and shipping catalog apps.

Clients can run in-house retail without Shopify while keeping the same
Docker/Django/Postgres stack and data-ownership promise. Closes #5.

site/pos_sync/apps.py

@@ -0,0 +1,12 @@
+from django.apps import AppConfig
+
+
+class PosSyncConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "pos_sync"
+    verbose_name = "POS API sync"
+
+    def ready(self):
+        from pos_sync import hooks
+
+        hooks.register()