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/shipping/urls.py

@@ -0,0 +1,13 @@
+from django.urls import path
+
+from shipping import views
+
+app_name = "shipping"
+
+urlpatterns = [
+    path("", views.shipment_list, name="shipment_list"),
+    path("new/", views.shipment_create, name="shipment_create"),
+    path("export/pirate-ship.csv", views.pirate_ship_export, name="pirate_ship_export"),
+    path("<uuid:pk>/", views.shipment_detail, name="shipment_detail"),
+    path("<uuid:pk>/buy/", views.shipment_buy, name="shipment_buy"),
+]