DebridDownloader receives directUrl from the server and downloads via
plain HTTPS with progress reporting, resume (Range), and pause/cancel.
- Add DirectURL, DirectFileName to agent Task and engine Task types
- Implement DebridDownloader: HTTPS download with progress, resume, cancel
- HTTP client with 30s ResponseHeaderTimeout
- Safe shortID helper to prevent slice panic on short IDs
- Validate 416 against Content-Range server size for resume integrity
- Register debridDl in daemon and one-shot download command
- Tests: available, download, resume, cancel, pause, fallback filename,
expired URL (410), unauthorized (401), shutdown, task propagation
