fix(backend): 商品编码改用最大序号生成不复用软删号 + 加 (shop_id,code) 唯一约束，根治重复编码

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YZ4DskSRKsSiheQonFtQvx

backend/internal/handler/product_test.go

@@ -2,13 +2,16 @@ package handler
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"gorm.io/gorm"
 
+	"github.com/wangjia/jiu/backend/internal/model"
 	"github.com/wangjia/jiu/backend/testutil"
 )
 
@@ -199,3 +202,67 @@ func TestProductHandler_Create_ShopIDFromToken(t *testing.T) {
 	dataBytes, _ := json.Marshal(data)
 	_ = dataBytes
 }
+
+// 自动编码按最大序号递增：P001 → P002 → P003。
+func TestProductHandler_AutoCode_Increment(t *testing.T) {
+	db := testutil.SetupTestDB()
+	shop := testutil.CreateTestShop(db, "AC001")
+	user := testutil.CreateTestUser(db, shop.ID, "admin", "pass", "admin")
+	token := getAuthToken(user.ID, shop.ID, "admin")
+	r := setupProtectedRouter(db)
+
+	var codes []string
+	for i := 0; i < 3; i++ {
+		w := makeRequest(r, "POST", "/api/v1/products", token, map[string]interface{}{
+			"name": fmt.Sprintf("AutoP %d", i), "unit": "个",
+		})
+		require.Equal(t, http.StatusCreated, w.Code)
+		data := parseResponse(w)["data"].(map[string]interface{})
+		codes = append(codes, data["code"].(string))
+	}
+	assert.Equal(t, []string{"P001", "P002", "P003"}, codes)
+}
+
+// 软删商品后，新建不复用被删的号（旧 count+1 逻辑会复用 → 重复，此为根因修复回归测试）。
+func TestProductHandler_AutoCode_NoReuseAfterSoftDelete(t *testing.T) {
+	db := testutil.SetupTestDB()
+	shop := testutil.CreateTestShop(db, "AC002")
+	user := testutil.CreateTestUser(db, shop.ID, "admin", "pass", "admin")
+	token := getAuthToken(user.ID, shop.ID, "admin")
+	r := setupProtectedRouter(db)
+
+	var lastID uint64
+	for i := 0; i < 3; i++ { // P001 P002 P003
+		w := makeRequest(r, "POST", "/api/v1/products", token, map[string]interface{}{
+			"name": fmt.Sprintf("NR %d", i), "unit": "个",
+		})
+		require.Equal(t, http.StatusCreated, w.Code)
+		lastID = extractID(w)
+	}
+	// 软删 P003
+	w := makeRequest(r, "DELETE", fmt.Sprintf("/api/v1/products/%d", lastID), token, nil)
+	require.Equal(t, http.StatusOK, w.Code)
+
+	// 再建 → 必须是 P004，不能复用已软删的 P003
+	w = makeRequest(r, "POST", "/api/v1/products", token, map[string]interface{}{
+		"name": "NR new", "unit": "个",
+	})
+	require.Equal(t, http.StatusCreated, w.Code)
+	data := parseResponse(w)["data"].(map[string]interface{})
+	assert.Equal(t, "P004", data["code"])
+}
+
+// (shop_id, code) 唯一约束生效，且重复被翻译成 gorm.ErrDuplicatedKey（Create/FindOrCreate 重试的前提）。
+func TestProductHandler_UniqueShopCode(t *testing.T) {
+	db := testutil.SetupTestDB()
+	shop := testutil.CreateTestShop(db, "UQ001")
+	require.NoError(t, db.Exec("CREATE UNIQUE INDEX uk_shop_code ON products(shop_id, code)").Error)
+
+	p1 := model.Product{TenantBase: model.TenantBase{ShopID: shop.ID}, Name: "A", Code: "P001"}
+	require.NoError(t, db.Create(&p1).Error)
+
+	p2 := model.Product{TenantBase: model.TenantBase{ShopID: shop.ID}, Name: "B", Code: "P001"}
+	err := db.Create(&p2).Error
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, gorm.ErrDuplicatedKey))
+}