feat(email): 初始化自建邮箱 MVP

- 新增 Go + SQLite 后端，支持登录、域名、邮箱、别名、联系人、规则、统计与邮件收发。
- 新增 Webmail 前端，支持多邮箱切换、邮件列表/阅读/写信、附件、搜索、主题切换与个人中心。
- 新增 Docker Compose 部署骨架，补充 Postfix、Dovecot、OpenDKIM、Nginx 配置与部署说明。

apps/api/cmd/server/main.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"log/slog"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"lanqin-email-api/internal/app"
+)
+
+func main() {
+	logger := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo}))
+	cfg := app.LoadConfig()
+
+	svc, err := app.New(cfg, logger)
+	if err != nil {
+		logger.Error("failed to initialize app", "error", err)
+		os.Exit(1)
+	}
+	defer svc.Close()
+
+	server := &http.Server{
+		Addr:              cfg.Addr,
+		Handler:           svc.Router(),
+		ReadHeaderTimeout: 10 * time.Second,
+	}
+
+	go func() {
+		logger.Info("LanQin API listening", "addr", cfg.Addr)
+		if err := server.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			logger.Error("server stopped unexpectedly", "error", err)
+			os.Exit(1)
+		}
+	}()
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+	<-ctx.Done()
+
+	shutdownCtx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	if err := server.Shutdown(shutdownCtx); err != nil {
+		logger.Error("server shutdown failed", "error", err)
+		os.Exit(1)
+	}
+	logger.Info("server stopped")
+}