feat(mall): restore real brand and sales facets

Wave 6, the last substantive piece of the mock-API migration. Wave 1 removed the
brand facet and the sales/comments sorts for want of a model; sales turn out to
be derivable from order_items and a brand model is a table plus a column.

- a `brands` table with a nullable `products.brand_id` and an ordered admin
  replace, mirroring categories and storefront content; a public `GET /api/brands`
  and a `brand_id` filter on the catalog, which the search page's facet uses
- `sold_count` per product, computed from `order_items` joined to orders that
  reached payment, so an abandoned or cancelled checkout cannot count as a sale.
  It is computed per read rather than stored, so it cannot drift from the orders
  that produced it
- `sort=sales` alongside `sort=price`; anything else is still a 400
- merchants can set a product's brand through the existing product upsert
- the review UI is gone: the card's review figure and the product detail page's
  reviews tab, summary and replies. There is no reviews model, and the mall
  attributed invented comments to named shoppers and showed a "good rate". The
  now-unreferenced fabrication helpers went with it (`salesOf`, `commentCountOf`,
  `commentsFor`, `commentStats`, `salesRankFor`, `productDetail`, `storeDetail`)

Two bugs found by checking rather than trusting: the fixed-data `listProducts`
had silently ignored `brand_id`, `sort` and `order`, so the restored facet
rendered but filtered nothing until the rollback check caught it; and the seed's
brand lookup read back through the shared `r` variable the product loop
reassigns, working once and then throwing.

Verified: 29 backend tests green including a new brand-and-sales case; all three
frontends build; searching filters by brand (24 to 6) and sorts by sales with
counts matching the API; a product page offers detail and after-sale tabs only,
with a real sold count; the fixed-data rollback filters by brand too.

OpenSpec change: openspec/changes/replace-mock-api-wave-6

apps/mall/components/ui/ProductCard.vue

@@ -1,14 +1,14 @@
 <script setup lang="ts">
 import type { Product } from "@vmall/shared";
 import { t as pick } from "@vmall/shared";
-import { commentCountOf, lowestSku, salesOf } from "~/mock/data";
+import { lowestSku } from "~/utils/product";
 
 const props = defineProps<{ product: Product }>();
 const { locale } = useI18n();
 
 const sku = computed(() => lowestSku(props.product));
-const sales = computed(() => salesOf(props.product));
-const comments = computed(() => commentCountOf(props.product));
+// Reported by the catalog API from paid orders; no reviews figure exists.
+const sold = computed(() => props.product.sold_count);
 </script>
 
 <template>
@@ -23,7 +23,7 @@ const comments = computed(() => commentCountOf(props.product));
         <span v-if="sku" class="price"><PriceText :amount-minor="sku.price_minor" :currency="sku.currency" /></span>
         <s v-if="sku" class="market"><PriceText :amount-minor="sku.price_minor + 10000" :currency="sku.currency" /></s>
       </p>
-      <p class="meta">{{ $t("product.salesCount", { n: sales }) }} · {{ $t("product.commentCount", { n: comments }) }}</p>
+      <p class="meta">{{ $t("product.salesCount", { n: sold }) }}</p>
     </div>
   </NuxtLink>
 </template>