feat: 媒体库 blob 去重存储与品牌/注册等前端调整

新增：
- blob_store 二进制去重存储 + blob_dedup 测试

其它：
- 后端 settings/auth/media_library/upload/operations 服务与 handler 调整
- 前端品牌（BrandLockup/BrandCropModal/BrandSeoPanel）、注册、媒体库、分析详情页、Header/SiteChrome 等更新

backend/service/operations_mail.go

@@ -24,6 +24,9 @@ import (
 
 type mailPayload struct{ To, Subject, Body string }
 
+// ErrEmailRegistered 注册发码时邮箱已被占用，handler 据此返回 400 业务提示
+var ErrEmailRegistered = errors.New("该邮箱已被注册，请直接登录")
+
 func (o *Operations) smtp(ctx context.Context, c MailConfig, p *mailPayload, id string) error {
 	password, e := o.open(c.Password, "mail:password")
 	if e != nil {
@@ -392,6 +395,16 @@ func (o *Operations) SendCode(email, purpose, ip string) (int, error) {
 			return 0, nil
 		}
 	}
+	if purpose == "register" {
+		// 入口拦截：邮箱已被占用则不发码，省一次邮件资源，也给用户明确提示
+		var n int64
+		if e = o.db.Model(&model.User{}).Where("LOWER(email) = ?", email).Count(&n).Error; e != nil {
+			return 0, e
+		}
+		if n > 0 {
+			return 0, ErrEmailRegistered
+		}
+	}
 	code := randomCode()
 	hash := counterKey(email + ":" + purpose + ":" + code)
 	e = o.db.Transaction(func(tx *gorm.DB) error {