feat: add migrate command, media server detection, and debrid auto-config

- Migration wizard from Sonarr/Radarr/Prowlarr (unarr migrate) [pre-beta]
  - Auto-detect instances via Docker, config files, port scan, Prowlarr
  - Import wanted list (monitored+missing movies/series)
  - Import download history and blocklist to avoid re-downloading
  - Extract debrid tokens from *arr download clients
  - Quality profile mapping to preferred_quality config
  - DISTINCT ON PostgreSQL query for optimal torrent selection
  - JSON export with --dry-run --json (text to stderr, JSON to stdout)
- Media server detection (Plex/Jellyfin/Emby) in unarr init
  - Detects library paths and offers them as download directory options
- Debrid auto-configuration in unarr init
  - Scans *arr instances for debrid tokens
  - Validates and saves via API if user confirms
- New preferred_quality setting in config (2160p/1080p/720p)
- Library scan command (unarr scan) with ffprobe metadata extraction

internal/engine/manager.go

@@ -280,6 +280,19 @@ func (m *Manager) processTask(ctx context.Context, task *Task) {
 	task.FilePath = finalPath
 	task.mu.Unlock()
 
+	// 4b. Handle upgrade replacement (mode = "upgrade")
+	if task.ReplacePath != "" {
+		backupDir := "" // uses default ~/.local/share/unarr/replaced/
+		if err := replaceFile(task.ReplacePath, finalPath, backupDir); err != nil {
+			log.Printf("[%s] replace warning: %v (keeping new file at %s)", task.ID[:8], err, finalPath)
+		} else {
+			task.mu.Lock()
+			task.FilePath = task.ReplacePath
+			task.mu.Unlock()
+			log.Printf("[%s] upgraded: replaced %s", task.ID[:8], task.ReplacePath)
+		}
+	}
+
 	// 5. Complete
 	if method == MethodTorrent && m.cfg.Organize.Enabled {
 		// Could add seeding here in the future