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/admin.py

@@ -0,0 +1,16 @@
+from django.contrib import admin
+
+from pos_sync.models import POSConnection, SyncEvent
+
+
+@admin.register(POSConnection)
+class POSConnectionAdmin(admin.ModelAdmin):
+    list_display = ("name", "provider", "is_active")
+    list_filter = ("provider", "is_active")
+
+
+@admin.register(SyncEvent)
+class SyncEventAdmin(admin.ModelAdmin):
+    list_display = ("direction", "sku", "quantity", "status", "created_at")
+    list_filter = ("direction", "status")
+    search_fields = ("sku", "external_id")