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/mock/api.ts

@@ -21,6 +21,7 @@ import type {
 import {
   BASE_CURRENCY,
   MOCK_BANNERS,
+  MOCK_BRANDS,
   MOCK_CATEGORIES,
   MOCK_CURRENCIES,
   MOCK_PROMOS,
@@ -162,7 +163,10 @@ export function createMockApi(): ApiClient {
         searchMockProducts({
           q: q.q,
           categoryId: q.category_id,
+          brandId: q.brand_id,
           shopId: q.shop_id,
+          sort: q.sort,
+          order: q.order,
           page: q.page,
           perPage: q.per_page,
         }),
@@ -175,6 +179,7 @@ export function createMockApi(): ApiClient {
     },
 
     listCategories: () => Promise.resolve(MOCK_CATEGORIES),
+    listBrands: () => Promise.resolve(MOCK_BRANDS),
     listCurrencies: () => Promise.resolve(MOCK_CURRENCIES),
     convert: (amountMinor, from, to) =>
       Promise.resolve({ amount_minor: mockConvertMinor(amountMinor, from, to), currency: to }),
@@ -375,8 +380,7 @@ export function createMockApi(): ApiClient {
         })),
       }),
 
-    listShops: (): Promise<ShopProfile[]> => Promise.resolve(MOCK_STORES.map(toShopProfile)),
-    getShop: (slug) => {
+    listShops: (): Promise<ShopProfile[]> => Promise.resolve(MOCK_STORES.map(toShopProfile)),    getShop: (slug) => {
       const store = storeById(slug);
       if (!store) return Promise.reject(new ApiError(404, "NOT_FOUND", "Shop not found"));
       return Promise.resolve(toShopProfile(store));
@@ -412,6 +416,8 @@ export function createMockApi(): ApiClient {
       getContent: () => unsupported(),
       replaceContent: () => unsupported(),
       setShopProfile: () => unsupported(),
+      getBrands: () => unsupported(),
+      replaceBrands: () => unsupported(),
     },
   };
 }