feat(search): 库存搜索支持拼音/首字母，按回车触发

- 新增 util.ToPinyin()，使用 go-pinyin 生成全拼和首字母
- Product model 新增 name_pinyin / name_initials 列（AutoMigrate）
- 启动时自动回填存量商品拼音
- Create / Update / FindOrCreate 写入时同步生成拼音
- 库存搜索 SQL 加入拼音/首字母 LIKE 条件
- 前端去掉 300ms 防抖，改为回车/点击搜索按钮触发

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/handler/inventory.go

@@ -64,9 +64,9 @@ func (h *InventoryHandler) List(c *gin.Context) {
 	inStock := c.Query("in_stock")
 
 	if keyword != "" {
-		baseWhere += " AND (COALESCE(NULLIF(p.name,''), inv.product_name) LIKE ? OR COALESCE(NULLIF(p.code,''), inv.product_code) LIKE ?)"
+		baseWhere += " AND (COALESCE(NULLIF(p.name,''), inv.product_name) LIKE ? OR COALESCE(NULLIF(p.code,''), inv.product_code) LIKE ? OR p.name_pinyin LIKE ? OR p.name_initials LIKE ?)"
 		like := "%" + keyword + "%"
-		args = append(args, like, like)
+		args = append(args, like, like, like, like)
 	}
 	if warehouseIDStr != "" {
 		baseWhere += " AND inv.warehouse_id = ?"