feat(mail): 增强邮件认证与审计能力

- 新增邮件认证结果解析、存储与前端展示，支持查看 SPF、DKIM、DMARC 及原始头信息。
- 扩展邮件规则条件，支持抄送、附件、大小、日期及嵌套条件组合。
- 增加邮箱容量统计与发送前配额校验，超限时返回明确错误。
- 新增管理端发送审计页面与接口，支持按邮箱、事件、Message-ID 和时间筛选。

apps/api/internal/app/maildir_sync.go

@@ -641,19 +641,20 @@ func (a *App) parseMaildirMessage(raw []byte, fallbackTo string) (storedMessage,
 		receivedAt = sentAt
 	}
 	return storedMessage{
-		MessageUID: newID("uid"),
-		MessageID:  strings.TrimSpace(m.Header.Get("Message-Id")),
-		Subject:    subject,
-		From:       from,
-		FromName:   fromName,
-		To:         to,
-		CC:         cc,
-		SentAt:     sentAt,
-		ReceivedAt: receivedAt,
-		Snippet:    snippetFrom(bodyText, bodyHTML),
-		BodyText:   bodyText,
-		BodyHTML:   bodyHTML,
-		IsRead:     false,
+		MessageUID:     newID("uid"),
+		MessageID:      strings.TrimSpace(m.Header.Get("Message-Id")),
+		Subject:        subject,
+		From:           from,
+		FromName:       fromName,
+		To:             to,
+		CC:             cc,
+		SentAt:         sentAt,
+		ReceivedAt:     receivedAt,
+		Snippet:        snippetFrom(bodyText, bodyHTML),
+		BodyText:       bodyText,
+		BodyHTML:       bodyHTML,
+		IsRead:         false,
+		Authentication: parseMailAuthentication(textproto.MIMEHeader(m.Header)),
 	}, parsed.Attachments, nil
 }