feat(admin): 品牌管理 + 新建商品

品牌管理：列出品牌及引用商品数，支持改名、合并重复品牌（把源品牌的商品并入目标后删除源）、删除未被引用的品牌。新建商品：商品列表新增「新建商品」入口，填写名称/条码/品牌/品类后创建并进入详情页继续补全。

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/internal/adminstore/write.go

@@ -170,6 +170,79 @@ ON CONFLICT (product_id) DO UPDATE SET
 	return after, nil
 }
 
+// ErrDuplicateGTIN is returned when a product GTIN already exists.
+var ErrDuplicateGTIN = errors.New("duplicate gtin")
+
+// CreateProduct inserts a new product from the admin UI. Only the core fields
+// are required; the operator completes the rest in the detail editor.
+func (s *Store) CreateProduct(ctx context.Context, actor string, in ProductInput) (*ProductDetail, error) {
+	if strings.TrimSpace(in.Name) == "" {
+		return nil, errors.New("name required")
+	}
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer tx.Rollback(ctx)
+
+	brandID := in.BrandID
+	if in.BrandName != nil && strings.TrimSpace(*in.BrandName) != "" {
+		bid, err := s.ensureBrand(ctx, tx, strings.TrimSpace(*in.BrandName))
+		if err != nil {
+			return nil, err
+		}
+		brandID = &bid
+	}
+
+	var gpc *string
+	if in.CategoryID != nil && *in.CategoryID != "" {
+		if err := tx.QueryRow(ctx, "SELECT gpc_brick_code FROM category WHERE id = $1", *in.CategoryID).Scan(&gpc); err != nil && !errors.Is(err, pgx.ErrNoRows) {
+			return nil, err
+		}
+	}
+
+	canonical, err := s.netCanonical(ctx, tx, in.NetContentValue, in.NetContentUnit)
+	if err != nil {
+		return nil, err
+	}
+
+	status := in.Status
+	if status == "" {
+		status = "active"
+	}
+
+	var id string
+	err = tx.QueryRow(ctx, `
+INSERT INTO product (gtin, name, brand_id, category_id, gpc_brick_code,
+   net_content_value, net_content_unit, net_content_canonical,
+   country_of_origin, status)
+VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10)
+RETURNING id`,
+		in.GTIN, in.Name, brandID, in.CategoryID, gpc,
+		in.NetContentValue, in.NetContentUnit, canonical,
+		in.CountryOfOrigin, status).Scan(&id)
+	if isUniqueViolation(err) {
+		return nil, ErrDuplicateGTIN
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	if _, err := s.recomputeQualityTx(ctx, tx, id); err != nil {
+		return nil, err
+	}
+	if err := tx.Commit(ctx); err != nil {
+		return nil, err
+	}
+
+	after, err := s.GetProduct(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	_ = s.writeAudit(ctx, actor, "create", "product", &id, []string{"name"}, nil, after)
+	return after, nil
+}
+
 func strEq(a, b *string) bool {
 	if a == nil && b == nil {
 		return true
@@ -330,15 +403,21 @@ func (s *Store) DeleteMSRP(ctx context.Context, productID, msrpID, actor string)
 
 // ---------- dictionaries ----------
 
-// Brand is a brand option for the edit form.
+// Brand is a brand option for the edit form and the management view.
 type Brand struct {
-	ID   string `json:"id"`
-	Name string `json:"name"`
+	ID           string `json:"id"`
+	Name         string `json:"name"`
+	ProductCount int    `json:"product_count"`
 }
 
-// ListBrands returns all brands ordered by name.
+// ListBrands returns all brands ordered by name, with the number of products
+// referencing each one.
 func (s *Store) ListBrands(ctx context.Context) ([]Brand, error) {
-	rows, err := s.pool.Query(ctx, "SELECT id, name FROM brand ORDER BY name")
+	rows, err := s.pool.Query(ctx, `
+SELECT b.id, b.name,
+       (SELECT count(*) FROM product p WHERE p.brand_id = b.id) AS product_count
+FROM brand b
+ORDER BY b.name`)
 	if err != nil {
 		return nil, err
 	}
@@ -346,7 +425,7 @@ func (s *Store) ListBrands(ctx context.Context) ([]Brand, error) {
 	out := []Brand{}
 	for rows.Next() {
 		var b Brand
-		if err := rows.Scan(&b.ID, &b.Name); err != nil {
+		if err := rows.Scan(&b.ID, &b.Name, &b.ProductCount); err != nil {
 			return nil, err
 		}
 		out = append(out, b)