fix(auth): SMTP 信封发件人用裸地址,修 501;发信失败记日志

两个真问题,前者被后者掩盖:
1. mailer.go 把带显示名的 SMTP_FROM(「穿山甲 <noreply@x>」)直接当 SMTP
   信封发件人(MAIL FROM)传给 smtp.SendMail,Resend 报 501 Bad sender address
   syntax → 验证码发不出。改用 net/mail 解析出裸地址作信封发件人,From: 头
   仍保留完整显示名。
2. service.go 两处异步发信把错误静默吞了(_ = SendCode/SendAlreadyRegistered),
   导致 SMTP 故障日志无痕、难排查(本次端口被封+501 都被吞)。改为失败时
   slog.Error 记日志,邮箱经 maskEmail 打码,绝不记验证码(守 no-secret-in-logs)。

测试:TestEnvelopeFrom(显示名/裸址/unicode → 裸址)守 501 回归;TestMaskEmail
守日志脱敏。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/auth/mailer.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"net/mail"
 	"net/smtp"
 	"strings"
 )
@@ -40,6 +41,18 @@ type SMTPMailer struct {
 // NewSMTPMailer builds an SMTPMailer.
 func NewSMTPMailer(cfg SMTPConfig) *SMTPMailer { return &SMTPMailer{cfg: cfg} }
 
+// envelopeFrom returns the bare address used for the SMTP envelope (MAIL FROM).
+// cfg.From may carry a display name ("穿山甲 <noreply@x>") for the From: header,
+// but the envelope sender must be a bare address — otherwise servers reject the
+// MAIL FROM with "501 Bad sender address syntax". Falls back to cfg.From when it
+// is already a bare address (ParseAddress still succeeds) or unparseable.
+func (m *SMTPMailer) envelopeFrom() string {
+	if a, err := mail.ParseAddress(m.cfg.From); err == nil {
+		return a.Address
+	}
+	return m.cfg.From
+}
+
 // SendCode delivers the verification code. The message body intentionally
 // contains no product-identifying or destination wording beyond a neutral
 // account-verification notice.
@@ -62,7 +75,7 @@ func (m *SMTPMailer) SendCode(_ context.Context, to, code string) error {
 
 	addr := fmt.Sprintf("%s:%d", m.cfg.Host, m.cfg.Port)
 	auth := smtp.PlainAuth("", m.cfg.Username, m.cfg.Password, m.cfg.Host)
-	if err := smtp.SendMail(addr, auth, m.cfg.From, []string{to}, []byte(msg)); err != nil {
+	if err := smtp.SendMail(addr, auth, m.envelopeFrom(), []string{to}, []byte(msg)); err != nil {
 		return fmt.Errorf("auth: smtp send: %w", err)
 	}
 	return nil
@@ -86,7 +99,7 @@ func (m *SMTPMailer) SendAlreadyRegistered(_ context.Context, to string) error {
 
 	addr := fmt.Sprintf("%s:%d", m.cfg.Host, m.cfg.Port)
 	auth := smtp.PlainAuth("", m.cfg.Username, m.cfg.Password, m.cfg.Host)
-	if err := smtp.SendMail(addr, auth, m.cfg.From, []string{to}, []byte(msg)); err != nil {
+	if err := smtp.SendMail(addr, auth, m.envelopeFrom(), []string{to}, []byte(msg)); err != nil {
 		return fmt.Errorf("auth: smtp send (already-registered): %w", err)
 	}
 	return nil