feat(backend): 入库每行新建独立产品发独立序列号(product 加生产日期/批次列)

入库 Create/Update 改为对每条明细新建独立 product（createIndependentProduct，
nextProductCode 自增 + uk_shop_code 唯一约束兜底），回填 product_id；不再按名称复用。
product 表加 production_date/batch_no。向后兼容：保留明细/库存快照列。

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

backend/internal/handler/stock_in_test.go

@@ -28,9 +28,11 @@ func TestStockInHandler_FullFlow(t *testing.T) {
 		"order_date":   time.Now().Format(time.RFC3339),
 		"items": []map[string]interface{}{
 			{
-				"product_id": product.ID,
-				"quantity":   10.0,
-				"unit_price": 5.0,
+				"product_name": "Test Beer",
+				"series":       "普通",
+				"spec":         "500ml",
+				"quantity":     10.0,
+				"unit_price":   5.0,
 			},
 		},
 	})
@@ -57,15 +59,17 @@ func TestStockInHandler_FullFlow(t *testing.T) {
 	w = makeRequest(r, "PUT", fmt.Sprintf("/api/v1/stock-in/orders/%d/approve", orderID), token, nil)
 	require.Equal(t, http.StatusOK, w.Code)
 
-	// 5. 验证库存变化
+	// 5. 验证库存变化：入库为明细新建了独立产品，库存指向它（不是预设的 product）
 	var inv model.Inventory
-	db.Where("shop_id = ? AND warehouse_id = ? AND product_id = ?",
-		shop.ID, warehouse.ID, product.ID).First(&inv)
+	db.Where("shop_id = ? AND warehouse_id = ?",
+		shop.ID, warehouse.ID).First(&inv)
 	assert.Equal(t, float64(10), inv.Quantity)
+	assert.NotZero(t, inv.ProductID)
+	assert.NotEqual(t, product.ID, inv.ProductID)
 
 	// 6. 验证库存流水
 	var logs []model.InventoryLog
-	db.Where("shop_id = ? AND product_id = ?", shop.ID, product.ID).Find(&logs)
+	db.Where("shop_id = ? AND product_id = ?", shop.ID, inv.ProductID).Find(&logs)
 	require.Len(t, logs, 1)
 	assert.Equal(t, "in", logs[0].Direction)
 	assert.Equal(t, float64(10), logs[0].Quantity)