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/client_site/urls.py

@@ -48,5 +48,19 @@ if apps.is_installed("social_ai"):
     urlpatterns += [
         path("portal/social/api/generate/", include("social_ai.urls")),
     ]
+if apps.is_installed("shop"):
+    urlpatterns += [
+        path("shop/", include("shop.public_urls")),
+        path("portal/shop/", include("shop.portal_urls")),
+    ]
+if apps.is_installed("pos_sync"):
+    urlpatterns += [path("portal/pos/", include("pos_sync.urls"))]
+if apps.is_installed("events"):
+    urlpatterns += [
+        path("events/", include("events.public_urls")),
+        path("portal/events/", include("events.portal_urls")),
+    ]
+if apps.is_installed("shipping"):
+    urlpatterns += [path("portal/shipping/", include("shipping.urls"))]
 
 handler404 = "public.views.page_not_found"