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/go.mod

@@ -0,0 +1,29 @@
+module lanqin-email-api
+
+go 1.22
+
+require (
+	github.com/go-chi/chi/v5 v5.1.0
+	github.com/microcosm-cc/bluemonday v1.0.27
+	golang.org/x/crypto v0.26.0
+	modernc.org/sqlite v1.31.1
+)
+
+require (
+	github.com/aymerick/douceur v0.2.0 // indirect
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/gorilla/css v1.0.1 // indirect
+	github.com/hashicorp/golang-lru/v2 v2.0.7 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v0.1.9 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	golang.org/x/net v0.26.0 // indirect
+	golang.org/x/sys v0.23.0 // indirect
+	modernc.org/gc/v3 v3.0.0-20240107210532-573471604cb6 // indirect
+	modernc.org/libc v1.55.3 // indirect
+	modernc.org/mathutil v1.6.0 // indirect
+	modernc.org/memory v1.8.0 // indirect
+	modernc.org/strutil v1.2.0 // indirect
+	modernc.org/token v1.1.0 // indirect
+)