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/gtin/gtin_test.go

@@ -0,0 +1,49 @@
+package gtin
+
+import "testing"
+
+func TestNormalizeValid(t *testing.T) {
+	cases := []struct{ in, want, typ string }{
+		{" 5449000000996 ", "5449000000996", "EAN13"}, // Coca-Cola EAN-13
+		{"3017624010701", "3017624010701", "EAN13"},   // Nutella EAN-13
+		{"036000291452", "036000291452", "UPC"},       // UPC-A
+		{"96385074", "96385074", "EAN8"},              // EAN-8
+		{"00012345600012", "00012345600012", "GTIN14"},
+		{"6901234567892", "6901234567892", "EAN13"}, // China 690 prefix
+	}
+	for _, c := range cases {
+		got, err := Normalize(c.in)
+		if err != nil {
+			t.Errorf("Normalize(%q) unexpected error: %v", c.in, err)
+			continue
+		}
+		if got != c.want {
+			t.Errorf("Normalize(%q) = %q, want %q", c.in, got, c.want)
+		}
+		if InferType(got) != c.typ {
+			t.Errorf("InferType(%q) = %q, want %q", got, InferType(got), c.typ)
+		}
+	}
+}
+
+func TestNormalizeRejects(t *testing.T) {
+	cases := []struct {
+		in   string
+		want error
+	}{
+		{"", ErrEmpty},
+		{"12ab5678", ErrFormat},
+		{"12345", ErrFormat},
+		{"5449000000997", ErrCheck},       // bad check digit
+		{"2012345678903", ErrRestricted}, // 20-29 in-store EAN-13
+		{"0212345678909", ErrRestricted}, // 02x variable measure
+		{"212345678909", ErrRestricted},  // UPC number system 2
+		{"02345673", ErrRestricted},      // EAN-8 in-store
+	}
+	for _, c := range cases {
+		_, err := Normalize(c.in)
+		if err != c.want {
+			t.Errorf("Normalize(%q) error = %v, want %v", c.in, err, c.want)
+		}
+	}
+}