feat(ecommerce): mensaje de urgencia por inventario real

Expone ProductVariant.inventoryQuantity (ya calculado desde
variant.inventory_quantity en Medusa, hoy colapsado al booleano
available) y lo usa para mostrar "Quedan N" cuando el stock real es
bajo (<=5), tanto en la tarjeta de producto como en la ficha. null
indica inventario no gestionado (backorder / manage_inventory=false),
donde nunca se muestra un numero. ADR 0005 seccion 3 (patron eBay).

Catalogo mock actualizado (data/products.json) con
inventoryQuantity=null en todas las variantes para seguir tipando
contra Product[] via `satisfies`.

workloads/ecommerce/components/product/product-card.tsx

@@ -2,12 +2,12 @@
 
 import Image from "next/image";
 import Link from "next/link";
-import { Heart, ShoppingBag, Sparkles } from "lucide-react";
+import { Heart, ShoppingBag, Sparkles, TriangleAlert } from "lucide-react";
 import { useState } from "react";
 import { Badge } from "@/components/ui/badge";
 import { Button } from "@/components/ui/button";
 import type { Product } from "@/lib/headless/types";
-import { formatMoney } from "@/lib/utils";
+import { formatMoney, LOW_STOCK_THRESHOLD } from "@/lib/utils";
 import { useCartStore } from "@/store/cart-store";
 import { useWishlistStore } from "@/store/wishlist-store";
 
@@ -23,6 +23,14 @@ export function ProductCard({ product, priority = false }: { product: Product; p
       : null;
 
   const [selectedVariantId, setSelectedVariantId] = useState(product.variants[0]?.id);
+  const selectedVariant = product.variants.find((variant) => variant.id === selectedVariantId) ?? product.variants[0];
+  const lowStock =
+    selectedVariant?.inventoryQuantity !== null &&
+    selectedVariant?.inventoryQuantity !== undefined &&
+    selectedVariant.inventoryQuantity > 0 &&
+    selectedVariant.inventoryQuantity <= LOW_STOCK_THRESHOLD
+      ? selectedVariant.inventoryQuantity
+      : null;
 
   return (
     <article className="group flex h-full flex-col overflow-hidden rounded-3xl border border-slate-100 bg-white shadow-lg shadow-slate-900/5 transition duration-300 hover:-translate-y-1.5 hover:shadow-2xl hover:shadow-violet-500/10">
@@ -56,6 +64,9 @@ export function ProductCard({ product, priority = false }: { product: Product; p
             ))}
           </div>
         )}
+        {lowStock !== null && (
+          <span className="flex items-center gap-1.5 text-xs font-bold text-amber-600"><TriangleAlert className="size-3.5" />Quedan {lowStock}</span>
+        )}
         <div className="mt-auto flex items-center justify-between gap-3 pt-1">
           <div className="flex flex-col">
             {product.compareAtPrice && <span className="text-xs font-bold text-slate-400 line-through">{formatMoney(product.compareAtPrice, product.currency)}</span>}