feat(daemon): add auto-scan, force start, and stall timeout default

- Auto-scan: daemon scans library daily (configurable via config.toml)
  [library] auto_scan = true, scan_interval = "24h"
- Force start: tasks with forceStart=true bypass concurrency semaphore
  (like Transmission's Force Start — opens temporary extra slot)
- Stall timeout default: 30m instead of unlimited, prevents dead torrents
  from permanently blocking download slots
- ForceStart field in agent.Task for CLI/server communication

internal/engine/manager.go

@@ -59,6 +59,17 @@ func (m *Manager) Submit(ctx context.Context, at agent.Task) {
 
 	m.reporter.Track(task)
 
+	// Force start: bypass semaphore (like Transmission's "Force Start")
+	if at.ForceStart {
+		log.Printf("[%s] force start: bypassing queue", task.ID[:8])
+		m.wg.Add(1)
+		go func() {
+			defer m.wg.Done()
+			m.processTask(ctx, task)
+		}()
+		return
+	}
+
 	// Acquire semaphore slot
 	select {
 	case m.sem <- struct{}{}:

internal/engine/torrent.go

@@ -82,8 +82,11 @@ type TorrentDownloader struct {
 
 // NewTorrentDownloader creates a BitTorrent downloader with a long-lived client.
 func NewTorrentDownloader(cfg TorrentConfig) (*TorrentDownloader, error) {
-	// 0 = unlimited for all timeouts (like qBittorrent)
-	// Users can set these in config.toml [downloads] section
+	// MetadataTimeout: 0 = unlimited (wait forever like qBittorrent)
+	// StallTimeout: default 30m (no bytes for 30 min = dead torrent, frees the slot)
+	if cfg.StallTimeout == 0 {
+		cfg.StallTimeout = 30 * time.Minute
+	}
 
 	if err := os.MkdirAll(cfg.DataDir, 0o755); err != nil {
 		return nil, fmt.Errorf("create data dir: %w", err)