Add shop, POS sync, event ticketing, and shipping catalog apps (#6)

## Summary
- Closes #5
- Optional `shop`, `pos_sync`, `events`, and `shipping` apps gated by `FEATURE_*` flags
- Deps match the catalog: shop/events need email + Stripe; POS/shipping need shop
- Portal inventory, POS webhooks, capacity tickets, EasyPost/Pirate Ship shipping

## Test plan
- [ ] `manage.py test` (152 passed locally)
- [ ] Shop cart + paid order decrements stocked inventory
- [ ] POS inbound webhook decrements SKU; paid order queues outbound reserve
- [ ] Event capacity blocks overbook; paid order emails ticket codes
- [ ] Shipping stub label + Pirate Ship CSV of unshipped paid orders
- [ ] `validate-env.sh` rejects shop without email/payments, POS/shipping without shop

Reviewed-on: #6

site/shop/portal_urls.py

@@ -0,0 +1,20 @@
+from django.urls import path
+
+from shop import views
+
+app_name = "shop_portal"
+
+urlpatterns = [
+    path("sales/", views.portal_sales, name="sales"),
+    path("products/", views.portal_product_list, name="product_list"),
+    path("products/new/", views.portal_product_edit, name="product_new"),
+    path("products/<uuid:pk>/", views.portal_product_edit, name="product_edit"),
+    path(
+        "products/<uuid:pk>/stock/",
+        views.portal_stock_adjust,
+        name="product_stock",
+    ),
+    path("orders/", views.portal_order_list, name="order_list"),
+    path("orders/<uuid:pk>/", views.portal_order_detail, name="order_detail"),
+    path("webhooks/stripe/", views.stripe_webhook, name="stripe_webhook"),
+]