New: Direct uploads & private storage

[peternaf]

• Direct file upload to Rendi. POST /v1/files/init-upload returns per-part presigned PUT URLs - upload chunks directly, then finalize with POST /v1/files/{file_id}/complete-upload.
  See the direct upload guide.
• Private storage and pre-signed urls. Set is_private: true on stored files or FFmpeg command
  outputs. The file lands in a private bucket and is only accessible via a time-limited presigned URL — request one by adding ?presigned_ttl_seconds=… on the poll or file-detail endpoints.

Also in this release: Free accounts can now store and upload files.