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/library/cache.go

@@ -0,0 +1,86 @@
+package library
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/torrentclaw/torrentclaw-cli/internal/config"
+)
+
+// CachePath returns the default library cache file path.
+func CachePath() string {
+	return filepath.Join(config.DataDir(), "library.json")
+}
+
+// LoadCache reads the library cache from disk. Returns nil if file doesn't exist.
+func LoadCache() (*LibraryCache, error) {
+	return LoadCacheFrom(CachePath())
+}
+
+// LoadCacheFrom reads the library cache from a specific path.
+func LoadCacheFrom(path string) (*LibraryCache, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("read cache: %w", err)
+	}
+
+	var cache LibraryCache
+	if err := json.Unmarshal(data, &cache); err != nil {
+		return nil, fmt.Errorf("parse cache: %w", err)
+	}
+
+	if cache.Version != cacheVersion {
+		return nil, nil // incompatible version, treat as missing
+	}
+
+	return &cache, nil
+}
+
+// SaveCache writes the library cache to disk atomically.
+func SaveCache(cache *LibraryCache) error {
+	return SaveCacheTo(cache, CachePath())
+}
+
+// SaveCacheTo writes the library cache to a specific path atomically.
+func SaveCacheTo(cache *LibraryCache, path string) error {
+	cache.Version = cacheVersion
+
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("create cache dir: %w", err)
+	}
+
+	data, err := json.MarshalIndent(cache, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encode cache: %w", err)
+	}
+
+	tmpPath := path + ".tmp"
+	if err := os.WriteFile(tmpPath, data, 0o644); err != nil {
+		return fmt.Errorf("write temp cache: %w", err)
+	}
+
+	if err := os.Rename(tmpPath, path); err != nil {
+		os.Remove(tmpPath)
+		return fmt.Errorf("rename cache: %w", err)
+	}
+
+	return nil
+}
+
+// BuildCacheIndex creates a lookup map from filePath → index for incremental scanning.
+func BuildCacheIndex(cache *LibraryCache) map[string]int {
+	if cache == nil {
+		return nil
+	}
+	idx := make(map[string]int, len(cache.Items))
+	for i, item := range cache.Items {
+		idx[item.FilePath] = i
+	}
+	return idx
+}