feat(deploy): 增加单容器部署方案

- 新增 `deploy/all-in-one` 构建与启动配置，将 API、Web、Nginx、Postfix、Dovecot、OpenDKIM 集成到一个容器中。
- 调整 `deploy/docker-compose.yml` 默认指向单容器部署，并补充分体调试编排 `deploy/docker-compose.stack.yml`。
- 更新 `README.md`、`deploy/README.md` 和 `.env.example`，同步说明新的部署方式与本地投递配置。

deploy/all-in-one/nginx.conf

@@ -0,0 +1,23 @@
+server {
+  listen 80;
+  server_name _;
+  root /usr/share/nginx/html;
+  index index.html;
+
+  location /api/ {
+    proxy_pass http://127.0.0.1:8080/api/;
+    proxy_http_version 1.1;
+    proxy_set_header Host $host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Proto $scheme;
+  }
+
+  location /healthz {
+    proxy_pass http://127.0.0.1:8080/healthz;
+  }
+
+  location / {
+    try_files $uri $uri/ /index.html;
+  }
+}