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_test.go

@@ -0,0 +1,99 @@
+package library
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSaveCacheAndLoad(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := filepath.Join(tmpDir, "library.json")
+
+	cache := &LibraryCache{
+		Version:   cacheVersion,
+		ScannedAt: "2026-03-29T10:00:00Z",
+		Path:      "/media/movies",
+		Items: []LibraryItem{
+			{
+				FilePath: "/media/movies/Inception.mkv",
+				FileName: "Inception.mkv",
+				FileSize: 5000000000,
+				ModTime:  "2026-01-15T12:00:00Z",
+				Title:    "Inception",
+				Year:     "2010",
+				Quality:  "1080p",
+			},
+		},
+	}
+
+	// Save
+	if err := SaveCacheTo(cache, path); err != nil {
+		t.Fatalf("SaveCacheTo: %v", err)
+	}
+
+	// Verify file exists
+	if _, err := os.Stat(path); err != nil {
+		t.Fatalf("cache file not found: %v", err)
+	}
+
+	// Load
+	loaded, err := LoadCacheFrom(path)
+	if err != nil {
+		t.Fatalf("LoadCacheFrom: %v", err)
+	}
+	if loaded == nil {
+		t.Fatal("loaded cache is nil")
+	}
+
+	if loaded.Version != cacheVersion {
+		t.Errorf("version = %d, want %d", loaded.Version, cacheVersion)
+	}
+	if loaded.Path != "/media/movies" {
+		t.Errorf("path = %q, want %q", loaded.Path, "/media/movies")
+	}
+	if len(loaded.Items) != 1 {
+		t.Fatalf("items count = %d, want 1", len(loaded.Items))
+	}
+	if loaded.Items[0].Title != "Inception" {
+		t.Errorf("title = %q, want %q", loaded.Items[0].Title, "Inception")
+	}
+}
+
+func TestLoadCacheNonExistent(t *testing.T) {
+	cache, err := LoadCacheFrom("/tmp/nonexistent-unarr-test.json")
+	if err != nil {
+		t.Fatalf("expected nil error, got: %v", err)
+	}
+	if cache != nil {
+		t.Fatalf("expected nil cache, got: %v", cache)
+	}
+}
+
+func TestBuildCacheIndex(t *testing.T) {
+	cache := &LibraryCache{
+		Items: []LibraryItem{
+			{FilePath: "/a.mkv"},
+			{FilePath: "/b.mkv"},
+			{FilePath: "/c.mkv"},
+		},
+	}
+
+	idx := BuildCacheIndex(cache)
+	if idx["/a.mkv"] != 0 {
+		t.Errorf("expected index 0 for /a.mkv, got %d", idx["/a.mkv"])
+	}
+	if idx["/b.mkv"] != 1 {
+		t.Errorf("expected index 1 for /b.mkv, got %d", idx["/b.mkv"])
+	}
+	if idx["/c.mkv"] != 2 {
+		t.Errorf("expected index 2 for /c.mkv, got %d", idx["/c.mkv"])
+	}
+}
+
+func TestBuildCacheIndexNil(t *testing.T) {
+	idx := BuildCacheIndex(nil)
+	if idx != nil {
+		t.Errorf("expected nil, got %v", idx)
+	}
+}