fix(deploy): 修复 Docker 部署两个问题

1. 后端 Dockerfile 开启 CGO（go-sqlite3 依赖），安装 gcc/musl-dev
2. nginx 添加 /auth/ 反向代理，修复注册登录 405 错误

frontend/nginx.conf

@@ -18,4 +18,13 @@ server {
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
     }
+
+    # 反向代理 /auth 到后端服务
+    location /auth/ {
+        proxy_pass http://backend:8080;
+        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;
+    }
 }