Add the step-by-step fixer and library types

Each pair is a Music (Artist/Album/01 Title) or Soundtracks (Album/01 Title)
library. The new Fixer tab takes the master, then the slave, through four
reviewed steps: folder structure, album tags (optional MusicBrainz lookup;
the slave reuses the fixed master's tags), folder names and file names,
then links the two libraries. Every fix is an undoable operation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/engine.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/cdnutter/playlink/internal/art"
 	"github.com/cdnutter/playlink/internal/fsops"
+	"github.com/cdnutter/playlink/internal/meta"
 	"github.com/cdnutter/playlink/internal/probe"
 	"github.com/cdnutter/playlink/internal/store"
 	"github.com/cdnutter/playlink/internal/tags"
@@ -49,6 +50,12 @@ type Engine struct {
 	artClient    *art.Client
 	artStore     *art.Store
 
+	meta *meta.Client
+
+	fixMu    sync.Mutex
+	fixPlans map[int64]*FixPlan // latest fixer check per pair
+	fixSeq   int64
+
 	plansMu sync.Mutex
 	plans   map[int64]*organizePlan // latest "rename now" preview per pair
 
@@ -309,6 +316,13 @@ func (e *Engine) NormalizePair(p *store.Pair) ([]store.Warning, error) {
 	if p.RetentionDays < 0 || p.ScanIntervalMinutes < 0 || p.SettleMinutes < 0 {
 		return nil, errors.New("intervals must not be negative")
 	}
+	switch p.LibraryType {
+	case "":
+		p.LibraryType = LibraryMusic
+	case LibraryMusic, LibrarySoundtracks:
+	default:
+		return nil, errors.New("library type must be music or soundtracks")
+	}
 	if err := normalizeOrganize(p); err != nil {
 		return nil, err
 	}