fix: 修复 Docker 环境中图片无法访问的问题

在 Nginx 配置中添加 /uploads 路径的代理规则，
将图片请求转发到后端容器的静态文件服务。

- 添加 location /uploads 代理配置
- 设置 7 天缓存策略提高性能

修复前: 图片上传后返回 404
修复后: 图片可以正常显示

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/nginx.docker.conf

@@ -28,6 +28,20 @@ server {
         proxy_cache_bypass $http_upgrade;
     }
 
+    # Upload files proxy to backend
+    location /uploads {
+        proxy_pass http://backend:13057/uploads;
+        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;
+
+        # Cache uploaded images
+        expires 7d;
+        add_header Cache-Control "public, immutable";
+    }
+
     # Static files with caching
     location /assets/ {
         expires 1y;