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/config/config.go

@@ -20,6 +20,7 @@ type Config struct {
 	Daemon        DaemonConfig        `toml:"daemon"`
 	Notifications NotificationsConfig `toml:"notifications"`
 	General       GeneralConfig       `toml:"general"`
+	Library       LibraryConfig       `toml:"library"`
 }
 
 type AuthConfig struct {
@@ -36,6 +37,7 @@ type AgentConfig struct {
 type DownloadConfig struct {
 	Dir              string `toml:"dir"`
 	PreferredMethod  string `toml:"preferred_method"`
+	PreferredQuality string `toml:"preferred_quality"` // "2160p", "1080p", "720p" — hint for auto-selection
 	MaxConcurrent    int    `toml:"max_concurrent"`
 	MaxDownloadSpeed string `toml:"max_download_speed"` // e.g. "10MB", "500KB", "0" = unlimited
 	MaxUploadSpeed   string `toml:"max_upload_speed"`   // e.g. "1MB", "0" = unlimited
@@ -62,6 +64,13 @@ type GeneralConfig struct {
 	NoColor bool   `toml:"no_color"`
 }
 
+type LibraryConfig struct {
+	ScanPath    string `toml:"scan_path"`    // remembered from last scan
+	Workers     int    `toml:"workers"`      // concurrent ffprobe (default 8)
+	FFprobePath string `toml:"ffprobe_path"` // optional explicit path
+	BackupDir   string `toml:"backup_dir"`   // for replaced files
+}
+
 // Default returns a Config with sensible defaults.
 func Default() Config {
 	return Config{