fix: resolve 8 architecture flaws across download, playlist, and webhook

- Add --continue and --max-filesize to yt-dlp for download resilience
- Route thumbnail downloads through HTTP_PROXY
- Show per-video progress (i/N) with cancel for playlist downloads
- Run playlist asynchronously so cancel callbacks can be processed
- Guard handlePlaylistConfirm against concurrent active jobs
- Close Done channel in playlist entry lifecycle
- Add stepState TTL (30 min) to prevent stale states
- Wrap webhook server for graceful shutdown on SIGTERM
- Refactor formatBytes to loop-based implementation
- Show first line of stderr in user-facing error messages
- Fix format pointer reuse in playlist loop
- Add CreatedAt field to stepState for TTL tracking
- Add MaxFileSize and ProgressPrefix fields to DownloadJob

internal/domain/types.go

@@ -124,8 +124,10 @@ type DownloadJob struct {
 	Speed          string
 	ETA            string
 	FileSize       int64
+	MaxFileSize    int64
 	FilePath       string
 	Title          string
+	ProgressPrefix string
 	QuotaApplied   bool
 	StderrLog      string
 	Done           chan struct{}