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/shop/admin.py

@@ -0,0 +1,24 @@
+from django.contrib import admin
+
+from shop.models import Order, OrderItem, Product
+
+
+@admin.register(Product)
+class ProductAdmin(admin.ModelAdmin):
+    list_display = ("name", "sku", "price", "stock_qty", "fulfillment", "is_published")
+    list_filter = ("fulfillment", "is_published")
+    search_fields = ("name", "sku")
+    prepopulated_fields = {"slug": ("name",)}
+
+
+class OrderItemInline(admin.TabularInline):
+    model = OrderItem
+    extra = 0
+
+
+@admin.register(Order)
+class OrderAdmin(admin.ModelAdmin):
+    list_display = ("number", "email", "amount", "status", "created_at")
+    list_filter = ("status",)
+    search_fields = ("number", "email", "customer_name")
+    inlines = [OrderItemInline]