Upload a CSV file to GCS imports prefix

Accepts a file via multipart/form-data and stores it in the configured Google Cloud Storage bucket under imports/<filename>, where <filename> is taken verbatim from the uploaded file's name. The upload is retried up to a configured number of attempts (default 3) with exponential backoff before failing. No CSV validation is performed server-side, and there is no overwrite protection: uploading a file whose name matches an existing object overwrites it. A successful upload emits no event and triggers no asynchronous job. Use this endpoint to stage a file in GCS before referencing it by path from GET /file/read or GET /file/download. Send a multipart/form-data request with a single field named exactly file, along with a valid JWT.