Get presigned upload URL

Returns a presigned URL for direct upload to cloud storage.

Flow:

1. Call this endpoint to get upload_url
2. PUT file directly to upload_url
3. Call POST /v1/inputs/{id}/complete to finalize

Notes:

• size is required to generate the presigned URL with correct content-length restriction
• Upload URL expires in 1 hour
• If upload not completed within expiry, resource transitions to failed status