fix: 批量改进 (#2/#23/#38/#39/#40/#47)

- Web: 隐藏 ICP 备案号（条件渲染），联系入口改为微信咨询（条件渲染，配置 wechat 字段后生效）
- backend: 拼音回填抽取为独立 cmd/backfill-pinyin 工具，不再在启动时运行
- backend: DB 连接池参数（max_idle_conns/max_open_conns）改为配置可控，默认 10/100
- backend: Inventory 反范式字段添加注释说明快照语义
- backend: 全量 handler 采用 util.RespondSuccess/RespondCreated 统一响应格式（51 处）

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

backend/cmd/backfill-pinyin/main.go

@@ -0,0 +1,57 @@
+// backfill-pinyin is a one-time migration tool that fills name_pinyin and
+// name_initials for products that have empty values.
+// Usage: go run ./cmd/backfill-pinyin
+package main
+
+import (
+	"log"
+	"strings"
+
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+
+	"github.com/spf13/viper"
+	"github.com/wangjia/jiu/backend/internal/model"
+	"github.com/wangjia/jiu/backend/internal/util"
+)
+
+func main() {
+	viper.SetConfigName("config")
+	viper.SetConfigType("yaml")
+	viper.AddConfigPath(".")
+	viper.AddConfigPath("./config")
+	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+	viper.AutomaticEnv()
+	_ = viper.BindEnv("database.dsn", "DATABASE_DSN")
+	if err := viper.ReadInConfig(); err != nil {
+		log.Println("[config] no config file, relying on env vars")
+	}
+
+	dsn := viper.GetString("database.dsn")
+	if dsn == "" {
+		log.Fatal("DATABASE_DSN is required")
+	}
+
+	db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Silent),
+	})
+	if err != nil {
+		log.Fatalf("failed to connect database: %v", err)
+	}
+
+	var products []model.Product
+	db.Where("name_pinyin = '' OR name_pinyin IS NULL").Find(&products)
+	if len(products) == 0 {
+		log.Println("No products to backfill.")
+		return
+	}
+	for i := range products {
+		full, initials := util.ToPinyin(products[i].Name)
+		db.Model(&products[i]).Updates(map[string]interface{}{
+			"name_pinyin":   full,
+			"name_initials": initials,
+		})
+	}
+	log.Printf("backfill-pinyin: updated %d products", len(products))
+}