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/apps.py

@@ -0,0 +1,12 @@
+from django.apps import AppConfig
+
+
+class ShopConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "shop"
+    verbose_name = "E-commerce"
+
+    def ready(self):
+        from shop import hooks
+
+        hooks.register()