feat(mail): 支持 SMTP Submission 由 API 接管

- 新增 `465/587` 提交服务与 TLS 配置，支持 StartTLS 和隐式 TLS。
- 提交时校验认证邮箱、权限和 `From` 一致性，先写入 `Sent` 再转发到后端 SMTP。
- 增加 `Message-ID` 去重与失败回滚，避免重复保存发送副本。
- 调整部署与文档，移除 Postfix 的 submission 监听，改由 LanQin API 对外提供提交端口。

apps/api/cmd/server/main.go

@@ -7,9 +7,12 @@ import (
 	"net/http"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 
+	smtpserver "github.com/emersion/go-smtp"
+
 	"lanqin-email-api/internal/app"
 )
 
@@ -29,6 +32,15 @@ func main() {
 		Handler:           svc.Router(),
 		ReadHeaderTimeout: 10 * time.Second,
 	}
+	submissionServers := &app.SubmissionServers{}
+	if strings.TrimSpace(cfg.SubmissionAddr) != "" || strings.TrimSpace(cfg.SubmissionTLSAddr) != "" {
+		tlsConfig, err := app.LoadServerTLSConfig(cfg)
+		if err != nil {
+			logger.Error("failed to initialize TLS config", "error", err)
+			os.Exit(1)
+		}
+		submissionServers = svc.NewSubmissionServers(tlsConfig)
+	}
 
 	go func() {
 		logger.Info("LanQin API listening", "addr", cfg.Addr)
@@ -37,6 +49,24 @@ func main() {
 			os.Exit(1)
 		}
 	}()
+	if submissionServers.Plain != nil {
+		go func() {
+			logger.Info("LanQin SMTP submission listening", "addr", cfg.SubmissionAddr)
+			if err := submissionServers.Plain.ListenAndServe(); err != nil && !errors.Is(err, smtpserver.ErrServerClosed) {
+				logger.Error("smtp submission server stopped unexpectedly", "error", err)
+				os.Exit(1)
+			}
+		}()
+	}
+	if submissionServers.TLS != nil {
+		go func() {
+			logger.Info("LanQin SMTP implicit TLS submission listening", "addr", cfg.SubmissionTLSAddr)
+			if err := submissionServers.TLS.ListenAndServeTLS(); err != nil && !errors.Is(err, smtpserver.ErrServerClosed) {
+				logger.Error("smtp tls submission server stopped unexpectedly", "error", err)
+				os.Exit(1)
+			}
+		}()
+	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
@@ -48,5 +78,9 @@ func main() {
 		logger.Error("server shutdown failed", "error", err)
 		os.Exit(1)
 	}
+	if err := submissionServers.Shutdown(shutdownCtx); err != nil {
+		logger.Error("smtp submission shutdown failed", "error", err)
+		os.Exit(1)
+	}
 	logger.Info("server stopped")
 }