fix: 修复 Docker Nginx 缺少 client_max_body_size 导致上传文件受限

- 添加 client_max_body_size 800m（与生产 Nginx 配置对齐）
- 添加 proxy_request_buffering off（避免大文件缓冲问题）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

infra/docker/nginx.conf

@@ -10,6 +10,8 @@ server {
     gzip_min_length 1024;
     gzip_types text/plain text/css text/xml text/javascript application/javascript application/json application/xml+rss;
 
+    client_max_body_size 800m;
+
     # SPA routing - all routes to index.html
     location / {
         try_files $uri $uri/ /index.html;
@@ -22,6 +24,7 @@ server {
         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;
+        proxy_request_buffering off;
     }
 
     # Cache static assets