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/events/admin.py

@@ -0,0 +1,24 @@
+from django.contrib import admin
+
+from events.models import Event, Ticket, TicketOrder
+
+
+@admin.register(Event)
+class EventAdmin(admin.ModelAdmin):
+    list_display = ("title", "starts_at", "capacity", "price", "is_published")
+    list_filter = ("is_published",)
+    search_fields = ("title", "venue")
+    prepopulated_fields = {"slug": ("title",)}
+
+
+@admin.register(TicketOrder)
+class TicketOrderAdmin(admin.ModelAdmin):
+    list_display = ("number", "event", "email", "quantity", "status")
+    list_filter = ("status",)
+    search_fields = ("number", "email")
+
+
+@admin.register(Ticket)
+class TicketAdmin(admin.ModelAdmin):
+    list_display = ("code", "event", "order")
+    search_fields = ("code",)