清除业务契约双读：抽奖/草稿/时间戳/邮件只认新格式，并删除 legacy 短代码改写。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/operations_mail.go

@@ -113,9 +113,6 @@ func (o *Operations) smtp(ctx context.Context, c MailConfig, p *mailPayload, id
 
 const defaultMailSubject = "{{site_name}} · {{purpose}}"
 
-// 旧版内置正文：读取时若仍是此值，升级为新默认模板。
-const legacyMailBody = "{{logo}}<h1>{{site_name}} · {{purpose}}</h1><p>您的验证码：</p><p><strong>{{code}}</strong></p><p>15 分钟内有效。如果不是您本人操作，请忽略本邮件。</p><p><a href=\"{{link}}\">前往站点完成{{purpose}}</a>（请粘贴以上验证码）</p>"
-
 const defaultMailBody = `<div class="email-wrap" style="margin:0;padding:32px 16px;background:#f5f7fb;font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,'Helvetica Neue',Arial,sans-serif;line-height:1.6;color:#1e293b;">
 <style>
 .email-wrap{margin:0;padding:32px 16px;background:#f5f7fb;font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,'Helvetica Neue',Arial,sans-serif;line-height:1.6;color:#1e293b;}
@@ -158,8 +155,7 @@ func (c *MailConfig) applyTemplateDefaults() {
 	if strings.TrimSpace(c.SubjectTemplate) == "" {
 		c.SubjectTemplate = defaultMailSubject
 	}
-	body := strings.TrimSpace(c.BodyTemplate)
-	if body == "" || body == legacyMailBody {
+	if strings.TrimSpace(c.BodyTemplate) == "" {
 		c.BodyTemplate = defaultMailBody
 	}
 }