fix(upload): 分片改 POST 并放宽 Nginx 反代，避免 PUT 大 body 断连

- 管理端分片请求改为 POST；后端同时保留 PUT
- /api/ 增加 proxy_request_buffering off；CORS Allow-Headers 略扩展

Made-with: Cursor

admin/src/api/admin.js

@@ -104,7 +104,7 @@ export const getMultipartUploadStatus = (siteId, uploadId) =>
   request.get(`/admin/sites/${siteId}/assets/multipart/${uploadId}/status`, { timeout: 60000 })
 
 export const putMultipartChunk = (siteId, uploadId, chunkIndex, blob) =>
-  request.put(`/admin/sites/${siteId}/assets/multipart/${uploadId}/chunk/${chunkIndex}`, blob, {
+  request.post(`/admin/sites/${siteId}/assets/multipart/${uploadId}/chunk/${chunkIndex}`, blob, {
     headers: { 'Content-Type': 'application/octet-stream' },
     timeout: 180000
   })