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_test.go

@@ -589,10 +589,11 @@ func TestMailTemplateGuards(t *testing.T) {
 	if cfg.BodyTemplate != defaultMailBody || cfg.SubjectTemplate != defaultMailSubject {
 		t.Fatal("defaults not applied")
 	}
-	cfg.BodyTemplate = legacyMailBody
+	customBody := "<p>{{code}}</p>"
+	cfg.BodyTemplate = customBody
 	cfg.applyTemplateDefaults()
-	if cfg.BodyTemplate != defaultMailBody {
-		t.Fatal("legacy default not upgraded")
+	if cfg.BodyTemplate != customBody {
+		t.Fatal("自定义正文不应被默认模板覆盖")
 	}
 	o := &Operations{cfg: &config.Config{SiteURL: "https://forum.example.com", DevMode: true}}
 	if e := o.validate(nil, "mail", &MailConfig{Port: 465, TLS: "tls", Timeout: 10, Retention: 30, BodyTemplate: "<script></script><p>no code</p>"}); e == nil || !strings.Contains(e.Error(), "{{code}}") {