feat(barcode): 多条码管理（后端+迁移+GTIN校验）WIP

- 迁移 0007: 新增 product_barcode 表（一品多码），回填旧 product.gtin 为主码，
  全局唯一索引保证「一码一品」，每品至多一个主码
- internal/gtin: GS1 GTIN-8/12/13/14 校验（校验位 + 拒收店内码/变量重量码/优惠券码）
- 公开只读 API: 任一条码命中商品、详情返回 barcodes、搜索匹配条码
- adminstore: 商品详情含 barcodes；新增 AddBarcode/DeleteBarcode/SetPrimaryBarcode，
  一码命中其他商品返回 ConflictError 供后台去重

待办（按用户要求暂停）: 后台 handler 路由、投稿/审核多条码、前后端 UI

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

api/internal/store/store.go

@@ -29,6 +29,15 @@ func (s *Store) Ping(ctx context.Context) error {
 	return s.pool.Ping(ctx)
 }
 
+// Barcode is one GS1 trade item number attached to a product.
+type Barcode struct {
+	GTIN      string  `json:"gtin"`
+	GTINType  string  `json:"gtin_type"`
+	PackLevel string  `json:"pack_level"`
+	Region    *string `json:"region"`
+	IsPrimary bool    `json:"is_primary"`
+}
+
 // Product is the full public view of a product.
 type Product struct {
 	ID              string         `json:"id"`
@@ -41,6 +50,7 @@ type Product struct {
 	NetContentUnit  *string        `json:"net_content_unit"`
 	CountryOfOrigin *string        `json:"country_of_origin"`
 	QualityScore    float64        `json:"quality_score"`
+	Barcodes        []Barcode      `json:"barcodes"`
 	Nutriments      map[string]any `json:"nutriments,omitempty"`
 	NutritionBasis  *string        `json:"nutrition_basis,omitempty"`
 	NutriScore      *string        `json:"nutri_score,omitempty"`
@@ -49,6 +59,27 @@ type Product struct {
 	Additives       []string       `json:"additives,omitempty"`
 }
 
+// ProductBarcodes returns every barcode attached to a product, primary first.
+func (s *Store) ProductBarcodes(ctx context.Context, productID string) ([]Barcode, error) {
+	rows, err := s.pool.Query(ctx,
+		`SELECT gtin, gtin_type, pack_level, region, is_primary
+		 FROM product_barcode WHERE product_id = $1
+		 ORDER BY is_primary DESC, gtin`, productID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []Barcode{}
+	for rows.Next() {
+		var b Barcode
+		if err := rows.Scan(&b.GTIN, &b.GTINType, &b.PackLevel, &b.Region, &b.IsPrimary); err != nil {
+			return nil, err
+		}
+		out = append(out, b)
+	}
+	return out, rows.Err()
+}
+
 // ProductSummary is a lightweight row used in search/listing responses.
 type ProductSummary struct {
 	ID           string  `json:"id"`
@@ -86,16 +117,35 @@ func scanProduct(row pgx.Row) (*Product, error) {
 	return &p, nil
 }
 
-// ProductByGTIN looks up an active product by its barcode.
+// ProductByGTIN looks up an active product by any of its barcodes.
 func (s *Store) ProductByGTIN(ctx context.Context, gtin string) (*Product, error) {
-	row := s.pool.QueryRow(ctx, productSelect+" WHERE p.gtin = $1 AND p.status = 'active'", gtin)
-	return scanProduct(row)
+	row := s.pool.QueryRow(ctx, productSelect+`
+ WHERE p.status = 'active'
+   AND (p.gtin = $1 OR EXISTS (
+         SELECT 1 FROM product_barcode pb
+         WHERE pb.product_id = p.id AND pb.gtin = $1))
+ LIMIT 1`, gtin)
+	p, err := scanProduct(row)
+	if err != nil {
+		return nil, err
+	}
+	if p.Barcodes, err = s.ProductBarcodes(ctx, p.ID); err != nil {
+		return nil, err
+	}
+	return p, nil
 }
 
 // ProductByID looks up a product by its UUID.
 func (s *Store) ProductByID(ctx context.Context, id string) (*Product, error) {
 	row := s.pool.QueryRow(ctx, productSelect+" WHERE p.id = $1", id)
-	return scanProduct(row)
+	p, err := scanProduct(row)
+	if err != nil {
+		return nil, err
+	}
+	if p.Barcodes, err = s.ProductBarcodes(ctx, p.ID); err != nil {
+		return nil, err
+	}
+	return p, nil
 }
 
 // SearchProducts performs a fuzzy name search with optional category subtree filter.
@@ -104,7 +154,9 @@ func (s *Store) SearchProducts(ctx context.Context, q, category string, limit, o
 	where := "WHERE p.status = 'active'"
 	if q != "" {
 		args = append(args, q)
-		where += " AND p.name ILIKE '%' || $1 || '%'"
+		where += ` AND (p.name ILIKE '%' || $1 || '%'
+			OR EXISTS (SELECT 1 FROM product_barcode pb
+			           WHERE pb.product_id = p.id AND pb.gtin ILIKE '%' || $1 || '%'))`
 	}
 	if category != "" {
 		args = append(args, category)