Changes
- Harden provider configuration for long-lived workers.
- Fail fast on conflicting provider environment collisions.
- Reject provider names that normalize to an empty rclone remote name.
- Add instance-scoped flags, envs, and timeouts.
- Generate unique temporary local provider names for upload/download helpers.
- Create automatic download temp directories with owner-only permissions.
- Add SFTP private_key/key_pem support and reject ambiguous key_pem + key_file configs.
- Include configuration/security tests in the fast test path.