fix(backend): 库存导入改用 shakinm/xlsReader，按唯一编号去重，入库日期取自 Excel

extrame/xls 误读真实 .xls（3264 行只读出 455 行、编号丢失、数字被当
日期序列），换用 shakinm/xlsReader 正确读取全部行。去重改为「有商品编号
时仅按编号匹配」，避免不同批次被 NSS 回退合并；Inventory.ProductCode
存行自身唯一编号。入库时间取 Excel 入库日期列写入 created_at。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/cmd/test_xls/main.go

@@ -1,71 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"strings"
-
-	xls "github.com/extrame/xls"
-)
-
-func safeXlsRow(sheet *xls.WorkSheet, r int) (row *xls.Row) {
-	defer func() { recover() }()
-	return sheet.Row(r)
-}
-
-func main() {
-	wb, err := xls.Open("/Users/wangjia/.claude/jobs/4f07558d/tmp/test_150rows.xls", "utf-8")
-	if err != nil {
-		fmt.Println("open error:", err)
-		return
-	}
-	sheet := wb.GetSheet(0)
-	if sheet == nil {
-		fmt.Println("no sheet")
-		return
-	}
-	fmt.Printf("MaxRow: %d\n", sheet.MaxRow)
-
-	numCols := 0
-	headerRow := sheet.Row(0)
-	fmt.Printf("Header LastCol: %d\n", headerRow.LastCol())
-	for c := 0; c < headerRow.LastCol(); c++ {
-		v := strings.TrimSpace(headerRow.Col(c))
-		if v != "" {
-			numCols = c + 1
-		}
-	}
-	if numCols == 0 {
-		numCols = 20
-	}
-	fmt.Printf("numCols: %d\n", numCols)
-
-	const maxRows = 100000
-	const maxEmpty = 5
-	emptyStreak := 0
-	rowCount := 0
-	for r := 0; r < maxRows; r++ {
-		row := safeXlsRow(sheet, r)
-		cells := make([]string, numCols)
-		isEmpty := true
-		if row != nil {
-			for c := 0; c < numCols; c++ {
-				cells[c] = strings.TrimSpace(row.Col(c))
-				if cells[c] != "" {
-					isEmpty = false
-				}
-			}
-		}
-		if isEmpty {
-			emptyStreak++
-			if emptyStreak >= maxEmpty {
-				fmt.Printf("Breaking at r=%d after %d empty streak\n", r, emptyStreak)
-				break
-			}
-			rowCount++
-			continue
-		}
-		emptyStreak = 0
-		rowCount++
-	}
-	fmt.Printf("Total rows read: %d\n", rowCount)
-}