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/util/pinyin.go

@@ -0,0 +1,48 @@
+package util
+
+import (
+	"strings"
+
+	gp "github.com/mozillazg/go-pinyin"
+)
+
+var (
+	fullArgs = func() gp.Args {
+		a := gp.NewArgs()
+		a.Style = gp.Normal
+		a.Fallback = func(r rune, _ gp.Args) []string {
+			return []string{strings.ToLower(string(r))}
+		}
+		return a
+	}()
+	initialArgs = func() gp.Args {
+		a := gp.NewArgs()
+		a.Style = gp.FirstLetter
+		a.Fallback = func(r rune, _ gp.Args) []string {
+			s := strings.ToLower(string(r))
+			if s == "" {
+				return nil
+			}
+			return []string{string(s[0])}
+		}
+		return a
+	}()
+)
+
+// ToPinyin converts a Chinese name to full pinyin and initials.
+// Non-Chinese characters are kept as-is (lowercased).
+// Example: "茅台酒" → ("maotaijiu", "mtj")
+func ToPinyin(s string) (full, initials string) {
+	var fb, ib strings.Builder
+	for _, row := range gp.Pinyin(s, fullArgs) {
+		if len(row) > 0 {
+			fb.WriteString(row[0])
+		}
+	}
+	for _, row := range gp.Pinyin(s, initialArgs) {
+		if len(row) > 0 {
+			ib.WriteString(row[0])
+		}
+	}
+	return fb.String(), ib.String()
+}