Uploads a file to storage.

Streams a single multipart file into object storage and returns an opaque `FileLink` (file ID plus content type) that can later be passed into agent inputs, attachments, or fetched via `GET StorageFileParam`. Max upload size is 50 MiB; larger requests fail at the request-size limit before reaching the handler. Concurrency-rate-limited per caller.

curl -X POST "https://api.fruxon.com/v1/tenants/string/storage"

{
  "fileId": "string",
  "fileType": "string",
  "promptLink": "<string:string/>"
}

{
  "type": "string",
  "title": "string",
  "status": 0,
  "detail": "string",
  "instance": "string",
  "property1": null,
  "property2": null
}