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/dl/ytdlp.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -305,6 +306,14 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 	// --newline uses \n instead of \r so bufio.Scanner can read line-by-line
 	args = append(args, "--progress", "--newline")
 
+	// Allow resuming partial downloads
+	args = append(args, "--continue")
+
+	// Enforce max file size limit
+	if job.MaxFileSize > 0 {
+		args = append(args, "--max-filesize", strconv.FormatInt(job.MaxFileSize, 10))
+	}
+
 	// Language selection if specified
 	if job.Language != "" {
 		args = append(args, "--sub-langs", job.Language)