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/hooks.py

@@ -0,0 +1,53 @@
+from core.registry import (
+    register_dashboard_collector,
+    register_feature,
+    register_portal_nav,
+    register_public_nav,
+)
+
+
+def register() -> None:
+    register_feature("shop")
+    register_public_nav(section="shop", label="Shop", url_name="shop:list", order=30)
+    register_portal_nav(
+        section="shop_sales",
+        label="Sales",
+        url_name="shop_portal:sales",
+        group="Retail",
+        order=5,
+    )
+    register_portal_nav(
+        section="shop_products",
+        label="Products",
+        url_name="shop_portal:product_list",
+        group="Retail",
+        order=10,
+    )
+    register_portal_nav(
+        section="shop_orders",
+        label="Orders",
+        url_name="shop_portal:order_list",
+        group="Retail",
+        order=20,
+    )
+    register_dashboard_collector(_dashboard)
+
+
+def _dashboard(request) -> dict:
+    from shop.models import Order, Product
+    from shop.stats import sales_summary
+
+    sales = sales_summary()
+    return {
+        "open_shop_orders": Order.objects.filter(
+            status__in=[Order.Status.OPEN, Order.Status.PAID]
+        ).count(),
+        "low_stock_products": Product.objects.filter(
+            is_published=True,
+            track_inventory=True,
+            fulfillment=Product.Fulfillment.STOCKED,
+            stock_qty__lte=3,
+        ).count(),
+        "shop_sales_30d": sales["order_count"],
+        "shop_revenue_30d": sales["revenue"],
+    }