feat: 实现对外邮件投递（外发队列 + MX 直投 + DKIM + 退信 + 管理后台）

- 新增 internal/outbound 模块：MX 查询、SMTP 出站客户端（EHLO/STARTTLS/
  MAIL/RCPT/DATA/QUIT）、4xx 临时失败与 5xx 永久失败分类、8BITMIME 支持
- 新增 outbound_messages 队列表与 OutboundStore，后台 worker 指数退避重试
- 永久失败/超限退信到发件人收件箱，包含原因与目标收件人
- 外发邮件使用域名 DKIM 私钥签名（go-msgauth）
- SMTP 提交集成：认证用户可发外部收件人，MAIL FROM 必须等于登录用户邮箱，
  未认证外部投递明确拒绝（防开放中继）
- Web 发信集成：外部收件人自动入队，附件以 multipart/mixed + base64 编码
  加入邮件正文
- 每用户每分钟/每日发送限速（max_per_day=0 可禁用外部投递）
- 管理后台新增外发队列页面：状态统计、失败原因、手动重试/取消
- 新增 [outbound] 配置段并更新 README / todo.md

internal/outbound/sign.go

@@ -0,0 +1,67 @@
+package outbound
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+
+	"github.com/emersion/go-msgauth/dkim"
+)
+
+// SignDKIM signs a raw RFC 5322 message with the domain's DKIM private key.
+// When the private key is empty the message is returned unchanged (unsigned).
+// The signature covers the standard header fields present in the message.
+func SignDKIM(raw []byte, domainName, selector, privateKeyPEM string) ([]byte, error) {
+	if privateKeyPEM == "" {
+		return raw, nil
+	}
+	if domainName == "" {
+		return raw, fmt.Errorf("DKIM domain is empty")
+	}
+	if selector == "" {
+		selector = "default"
+	}
+
+	block, _ := pem.Decode([]byte(privateKeyPEM))
+	if block == nil {
+		return raw, fmt.Errorf("invalid DKIM private key PEM for %s", domainName)
+	}
+
+	signer, err := parseSigner(block)
+	if err != nil {
+		return raw, fmt.Errorf("parse DKIM private key for %s: %v", domainName, err)
+	}
+
+	var out bytes.Buffer
+	options := &dkim.SignOptions{
+		Domain:   domainName,
+		Selector: selector,
+		Signer:   signer,
+	}
+	if err := dkim.Sign(&out, bytes.NewReader(raw), options); err != nil {
+		return raw, fmt.Errorf("DKIM signing for %s failed: %v", domainName, err)
+	}
+	return out.Bytes(), nil
+}
+
+// parseSigner parses an RSA private key PEM block into a crypto.Signer.
+// Both PKCS#1 ("RSA PRIVATE KEY") and PKCS#8 ("PRIVATE KEY") are supported.
+func parseSigner(block *pem.Block) (crypto.Signer, error) {
+	// PKCS#1 is what the domain management form stores.
+	if key, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
+		return key, nil
+	}
+
+	// PKCS#8 fallback for externally generated keys.
+	parsed, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		return nil, err
+	}
+	signer, ok := parsed.(crypto.Signer)
+	if !ok {
+		return nil, fmt.Errorf("unsupported private key type %T", parsed)
+	}
+	return signer, nil
+}