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

packages/shared/src/api.ts

@@ -1,6 +1,8 @@
 import type {
   Address,
   AuthTokens,
+  Brand,
+  BrandInput,
   Cart,
   Category,
   ContentInputByKind,
@@ -44,15 +46,17 @@ export interface ProductListQuery {
   page?: number;
   per_page?: number;
   category_id?: string;
+  brand_id?: string;
   q?: string;
   shop_id?: string;
-  /** `price` orders by each product's lowest active SKU price. */
-  sort?: "price";
+  /** `price` orders by lowest active SKU price; `sales` by units sold. */
+  sort?: "price" | "sales";
   order?: "asc" | "desc";
 }
 
 export interface ProductUpsertBody {
   category_id?: string | null;
+  brand_id?: string | null;
   slug: string;
   name: LocalizedText;
   description?: LocalizedText;
@@ -145,6 +149,7 @@ export interface ApiClient {
   listProducts(q?: ProductListQuery): Promise<Paged<Product>>;
   getProduct(idOrSlug: string): Promise<Product>;
   listCategories(): Promise<Category[]>;
+  listBrands(): Promise<Brand[]>;
   listCurrencies(): Promise<Currency[]>;
   convert(amountMinor: number, from: string, to: string): Promise<ConvertResult>;
   getCart(): Promise<Cart>;
@@ -223,6 +228,7 @@ export function createApi(opts: ApiClientOptions): ApiClient {
     listProducts: (q = {}) => r("GET", "/products", undefined, { ...q }),
     getProduct: (idOrSlug) => r("GET", `/products/${idOrSlug}`),
     listCategories: () => r("GET", "/categories"),
+    listBrands: () => r("GET", "/brands"),
     listCurrencies: () => r("GET", "/currencies"),
     convert: (amountMinor, from, to) =>
       r("GET", "/currencies/convert", undefined, { amount_minor: amountMinor, from, to }),
@@ -282,6 +288,9 @@ export function createApi(opts: ApiClientOptions): ApiClient {
       getContent: () => r("GET", "/admin/content"),
       replaceContent: (kind, items) => r("PUT", `/admin/content/${kind}`, items),
       setShopProfile: (id, body) => r("PUT", `/admin/shops/${id}/profile`, body),
+    /** Replaces the whole ordered brand list. */
+    getBrands: () => r("GET", "/brands"),
+    replaceBrands: (items) => r("PUT", "/admin/brands", items),
     },
   };
 }

packages/shared/src/types.ts

@@ -50,15 +50,32 @@ export interface Product {
   id: string;
   shop_id: string;
   category_id: string | null;
+  brand_id: string | null;
   slug: string;
   name: LocalizedText;
   description: LocalizedText;
   images: string[];
   status: ProductStatus;
   created_at: string;
+  /** Units sold across paid orders; every product payload carries it. */
+  sold_count: number;
   skus?: Sku[];
 }
 
+export interface Brand {
+  id: string;
+  name: LocalizedText;
+  slug: string;
+  position: number;
+  active: boolean;
+}
+
+export interface BrandInput {
+  slug: string;
+  name: LocalizedText;
+  active?: boolean;
+}
+
 export interface Sku {
   id: string;
   product_id: string;