Add a shop sales dashboard with 30-day charts (#8)

## Summary
- Closes #7
- Portal Retail **Sales** page: 30-day orders/revenue/units/AOV, daily charts, top products
- Home dashboard cards for 30-day shop sales and revenue when `FEATURE_SHOP` is on
- Stacks on #6 (shop catalog). Merge that first, then retarget `master` if needed.

## Test plan
- [ ] Sales page requires login; empty state with no paid orders
- [ ] Paid/fulfilled in the last 30 days counted; draft/open/cancelled/old excluded
- [ ] Top products ranked by units; home dashboard shows 30-day sales count
- [ ] `manage.py test shop core.tests_features`

Reviewed-on: #8

site/shop/views.py

@@ -26,6 +26,7 @@ from shop.services import (
     save_cart,
     set_cart_qty,
 )
+from shop.stats import sales_dashboard
 
 logger = logging.getLogger(__name__)
 
@@ -222,6 +223,11 @@ def portal_stock_adjust(request, pk):
     return redirect("shop_portal:product_edit", pk=product.pk)
 
 
+@login_required
+def portal_sales(request):
+    return render(request, "shop/portal/sales.html", sales_dashboard())
+
+
 @login_required
 def portal_order_list(request):
     orders = Order.objects.all()[:200]