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/brand.go

@@ -0,0 +1,154 @@
+package adminstore
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// Brand-management errors, mapped to client statuses by the handler.
+var (
+	// ErrDuplicateBrand is returned when a brand name already exists.
+	ErrDuplicateBrand = errors.New("duplicate brand name")
+	// ErrBrandInUse blocks deleting a brand still referenced by products.
+	ErrBrandInUse = errors.New("brand in use")
+	// ErrInvalidMerge is returned when a merge target is missing or equal to
+	// the source.
+	ErrInvalidMerge = errors.New("invalid merge target")
+)
+
+// CreateBrand inserts a new brand. Names are unique by normalized form.
+func (s *Store) CreateBrand(ctx context.Context, actor, name string) (*Brand, error) {
+	name = strings.TrimSpace(name)
+	if name == "" {
+		return nil, errors.New("name required")
+	}
+	var b Brand
+	err := s.pool.QueryRow(ctx,
+		`INSERT INTO brand (name, normalized_name) VALUES ($1, $2) RETURNING id, name, 0`,
+		name, normBrand(name)).Scan(&b.ID, &b.Name, &b.ProductCount)
+	if isUniqueViolation(err) {
+		return nil, ErrDuplicateBrand
+	}
+	if err != nil {
+		return nil, err
+	}
+	_ = s.writeAudit(ctx, actor, "create", "brand", &b.ID, []string{"name"}, nil, b)
+	return &b, nil
+}
+
+// UpdateBrand renames a brand, keeping the normalized name in sync.
+func (s *Store) UpdateBrand(ctx context.Context, id, actor, name string) (*Brand, error) {
+	name = strings.TrimSpace(name)
+	if name == "" {
+		return nil, errors.New("name required")
+	}
+	ct, err := s.pool.Exec(ctx,
+		"UPDATE brand SET name = $1, normalized_name = $2 WHERE id = $3",
+		name, normBrand(name), id)
+	if isUniqueViolation(err) {
+		return nil, ErrDuplicateBrand
+	}
+	if err != nil {
+		return nil, err
+	}
+	if ct.RowsAffected() == 0 {
+		return nil, ErrNotFound
+	}
+	out, err := s.getBrand(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	_ = s.writeAudit(ctx, actor, "update", "brand", &id, []string{"name"}, nil, out)
+	return out, nil
+}
+
+// MergeBrands reassigns every product of src to dst, then deletes src. Useful
+// for collapsing duplicate brands (e.g. "可口可乐" and "Coca-Cola").
+func (s *Store) MergeBrands(ctx context.Context, srcID, dstID, actor string) (*Brand, error) {
+	if srcID == dstID {
+		return nil, ErrInvalidMerge
+	}
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer tx.Rollback(ctx)
+
+	var dstName string
+	err = tx.QueryRow(ctx, "SELECT name FROM brand WHERE id = $1", dstID).Scan(&dstName)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrInvalidMerge
+	}
+	if err != nil {
+		return nil, err
+	}
+	var srcName string
+	err = tx.QueryRow(ctx, "SELECT name FROM brand WHERE id = $1", srcID).Scan(&srcName)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	if _, err := tx.Exec(ctx, "UPDATE product SET brand_id = $1 WHERE brand_id = $2", dstID, srcID); err != nil {
+		return nil, err
+	}
+	if _, err := tx.Exec(ctx, "DELETE FROM brand WHERE id = $1", srcID); err != nil {
+		return nil, err
+	}
+	if err := tx.Commit(ctx); err != nil {
+		return nil, err
+	}
+
+	out, err := s.getBrand(ctx, dstID)
+	if err != nil {
+		return nil, err
+	}
+	_ = s.writeAudit(ctx, actor, "merge", "brand", &srcID, []string{"name"},
+		map[string]string{"name": srcName},
+		map[string]string{"merged_into": dstName, "merged_into_id": dstID})
+	return out, nil
+}
+
+// DeleteBrand removes a brand not referenced by any product.
+func (s *Store) DeleteBrand(ctx context.Context, id, actor string) error {
+	before, err := s.getBrand(ctx, id)
+	if err != nil {
+		return err
+	}
+	if before.ProductCount > 0 {
+		return fmt.Errorf("%w: %d products", ErrBrandInUse, before.ProductCount)
+	}
+	ct, err := s.pool.Exec(ctx, "DELETE FROM brand WHERE id = $1", id)
+	if err != nil {
+		if isForeignKeyViolation(err) {
+			return ErrBrandInUse
+		}
+		return err
+	}
+	if ct.RowsAffected() == 0 {
+		return ErrNotFound
+	}
+	_ = s.writeAudit(ctx, actor, "delete", "brand", &id, []string{"name"}, before, nil)
+	return nil
+}
+
+func (s *Store) getBrand(ctx context.Context, id string) (*Brand, error) {
+	var b Brand
+	err := s.pool.QueryRow(ctx, `
+SELECT b.id, b.name,
+       (SELECT count(*) FROM product p WHERE p.brand_id = b.id)
+FROM brand b WHERE b.id = $1`, id).Scan(&b.ID, &b.Name, &b.ProductCount)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &b, nil
+}