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/task.go

@@ -50,6 +50,8 @@ type Task struct {
 	DirectFileName  string // Original filename from direct URL
 	NzbID           string // Pre-resolved NZB ID (usenet)
 	NzbPassword     string // Password for encrypted NZB archives
+	ReplacePath     string // File to replace after download (upgrade mode)
+	LibraryItemID   int    // Library item being upgraded
 
 	// Runtime state
 	Status          TaskStatus
@@ -88,6 +90,8 @@ func NewTaskFromAgent(at agent.Task) *Task {
 		DirectFileName:  at.DirectFileName,
 		NzbID:           at.NzbID,
 		NzbPassword:     at.NzbPassword,
+		ReplacePath:     at.ReplacePath,
+		LibraryItemID:   at.LibraryItemID,
 		Mode:            mode,
 		Status:          StatusClaimed,
 		ClaimedAt:       time.Now(),