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/public/views.py

@@ -70,6 +70,10 @@ def sitemap_xml(request):
     from django.apps import apps as django_apps
     if django_apps.is_installed("blog"):
         paths.append(("blog:list", "0.7", "weekly"))
+    if django_apps.is_installed("shop"):
+        paths.append(("shop:list", "0.8", "weekly"))
+    if django_apps.is_installed("events"):
+        paths.append(("events:list", "0.8", "weekly"))
     urls = []
     for name, priority, changefreq in paths:
         path = reverse(name)