feat(16-03): WriteTrackTags pipeline with player safety, scan mutex, events, and app wiring

- TagWriter struct with WriteTrackTags entry point orchestrating full pipeline
- PlayerStopper interface + playerAdapter for import-cycle-free player safety
- PipelineLocker interface + pipelineMu on Library for scan/write mutual exclusion
- TrackMetadataChanged event constant + auto-generated TypeScript events
- TagWriter wired into app.go as Wails binding with SetContext in OnStartup
- 5 pipeline integration tests: player safety, scan mutex, orphan cleanup, genre relink, full DB sync
- Lint fixes: static error, else-if, wsl cuddle, golines

backend/app.go

@@ -22,6 +22,7 @@ import (
 	"yellowjacket/backend/playlist"
 	"yellowjacket/backend/profiling"
 	"yellowjacket/backend/queue"
+	"yellowjacket/backend/tagwriter"
 )
 
 // YellowJacketApp is the main application struct for Wails.
@@ -37,6 +38,7 @@ type YellowJacketApp struct {
 	playlist      *playlist.Service
 	queue         *queue.Queue
 	mediaControls mediacontrols.Handler
+	tagWriter     *tagwriter.TagWriter
 	appContext    context.Context
 	appConfig     *config.Config
 	startupErr    error
@@ -115,6 +117,14 @@ func NewYellowJacketApp(
 		yjApp.logger.WithGroup("player"), yjApp.database,
 	)
 
+	// create tag writer
+	yjApp.tagWriter = tagwriter.NewTagWriter(
+		yjApp.logger,
+		yjApp.database,
+		&playerAdapter{p: yjApp.player},
+		yjApp.library,
+	)
+
 	yjApp.FEBindings = []any{
 		yjApp.FrontendUtil,
 		yjApp.appConfig,
@@ -122,11 +132,22 @@ func NewYellowJacketApp(
 		yjApp.playlist,
 		yjApp.queue,
 		yjApp.player,
+		yjApp.tagWriter,
 	}
 
 	return yjApp, nil
 }
 
+// playerAdapter wraps *player.Player to satisfy the tagwriter.PlayerStopper
+// interface, breaking the import cycle between tagwriter and player.
+type playerAdapter struct{ p *player.Player }
+
+func (a *playerAdapter) CurrentFilePath() string {
+	return a.p.GetCurrentTrackInfo().FilePath
+}
+
+func (a *playerAdapter) StopAndRelease() { a.p.UnloadTrack() }
+
 // WindowConfig returns the window configuration for use by the host.
 func (yj *YellowJacketApp) WindowConfig() *config.WindowConfig {
 	return yj.appConfig.Window
@@ -157,6 +178,7 @@ func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 	}
 
 	yj.player.SetContext(ctx)
+	yj.tagWriter.SetContext(ctx)
 
 	// Wire queue (created in NewYellowJacketApp for Wails binding)
 	yj.queue.SetContext(ctx)

backend/events/events.go

@@ -67,3 +67,8 @@ const (
 	LibraryRenamed = "LibraryRenamed"
 	LibraryRemoved = "LibraryRemoved"
 )
+
+// Tag writing events.
+const (
+	TrackMetadataChanged = "TrackMetadataChanged"
+)

backend/library/library.go

@@ -110,6 +110,11 @@ type Library struct {
 	// scanHooks holds callbacks for post-scan processing
 	// (e.g. resolving phantom playlist tracks).
 	scanHooks ScanHooks
+
+	// pipelineMu provides mutual exclusion between the scan
+	// pipeline and the tag write pipeline.  Acquired at the
+	// start of each pipeline, released at the end.
+	pipelineMu sync.Mutex
 }
 
 // SetRescanHooks provides optional hooks for cross-cutting
@@ -157,6 +162,15 @@ func NewLibrary(
 	return library, nil
 }
 
+// AcquirePipelineLock acquires the pipeline mutex for a tag write
+// operation.  The caller must call ReleasePipelineLock when done.
+// If a scan is currently in progress, AcquirePipelineLock blocks
+// until it completes (and vice versa).
+func (l *Library) AcquirePipelineLock() { l.pipelineMu.Lock() }
+
+// ReleasePipelineLock releases the pipeline mutex after a tag write.
+func (l *Library) ReleasePipelineLock() { l.pipelineMu.Unlock() }
+
 // SetContext sets the Wails runtime context and registers event handlers.
 func (l *Library) SetContext(ctx context.Context) {
 	l.mu.Lock()
@@ -187,6 +201,10 @@ func (l *Library) scanInternal(
 	libraryName string,
 	libraryPath string,
 ) *ScanMetrics {
+	// Acquire pipeline lock for scan/write mutual exclusion.
+	l.pipelineMu.Lock()
+	defer l.pipelineMu.Unlock()
+
 	metrics := newScanMetrics()
 	metrics.LibraryID = libraryID
 	metrics.LibraryName = libraryName

backend/tagwriter/dbsync.go

@@ -259,8 +259,10 @@ func syncDatabase(
 	// 6. Update FTS5 search index (within the transaction).
 	// ------------------------------------------------------------------
 	newTitle := newName
+
 	newArtist := params.changes[FieldArtist]
 	artistStr := ""
+
 	if newArtist != nil {
 		artistStr, _ = newArtist.(string)
 	}
@@ -277,13 +279,11 @@ func syncDatabase(
 	newAlbum := ""
 	if v, ok := params.changes[FieldAlbum].(string); ok {
 		newAlbum = v
-	} else {
+	} else if len(params.oldRGLinks) > 0 {
 		// Look up current album from release groups if unchanged.
-		if len(params.oldRGLinks) > 0 {
-			rg, rgErr := txq.GetReleaseGroup(ctx, params.oldRGLinks[0].ReleaseGroupID)
-			if rgErr == nil {
-				newAlbum = rg.Name
-			}
+		rg, rgErr := txq.GetReleaseGroup(ctx, params.oldRGLinks[0].ReleaseGroupID)
+		if rgErr == nil {
+			newAlbum = rg.Name
 		}
 	}
 

backend/tagwriter/pipeline.go

@@ -0,0 +1,174 @@
+package tagwriter
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"time"
+
+	wailsruntime "github.com/wailsapp/wails/v2/pkg/runtime"
+
+	"yellowjacket/backend/database"
+	"yellowjacket/backend/events"
+)
+
+// errNoChanges is returned when WriteTrackTags is called with an
+// empty TagChanges map.
+var errNoChanges = errors.New("tagwriter: no changes provided")
+
+// PlayerStopper checks whether a file is currently playing and
+// stops playback if needed.  Defined as an interface to break the
+// import cycle between tagwriter and player.
+type PlayerStopper interface {
+	// CurrentFilePath returns the file path of the currently-
+	// loaded track, or empty string if nothing is loaded.
+	CurrentFilePath() string
+	// StopAndRelease stops playback and releases the file handle.
+	StopAndRelease()
+}
+
+// PipelineLocker abstracts the library's pipeline mutex for
+// scan/write mutual exclusion.
+type PipelineLocker interface {
+	AcquirePipelineLock()
+	ReleasePipelineLock()
+}
+
+// TagWriter orchestrates the complete tag writing pipeline:
+// file write → DB sync → event emission.
+type TagWriter struct {
+	logger  *slog.Logger
+	db      *database.DB
+	ctx     context.Context // Wails context for event emission
+	player  PlayerStopper
+	library PipelineLocker
+}
+
+// NewTagWriter creates a TagWriter with the given dependencies.
+// Call SetContext after the Wails runtime is available.
+func NewTagWriter(
+	logger *slog.Logger,
+	db *database.DB,
+	player PlayerStopper,
+	library PipelineLocker,
+) *TagWriter {
+	return &TagWriter{
+		logger:  logger.WithGroup("tagwriter"),
+		db:      db,
+		player:  player,
+		library: library,
+	}
+}
+
+// SetContext stores the Wails runtime context for event emission.
+// Called during the two-phase init pattern in OnStartup.
+func (tw *TagWriter) SetContext(ctx context.Context) {
+	tw.ctx = ctx
+}
+
+// WriteTrackTags is the single entry point for writing metadata to
+// a track's audio file and synchronising all changes to the
+// database.  It accepts a track ID (audio_file.id) and a diff map
+// of changed fields.
+//
+// The pipeline:
+//  1. Look up track from DB.
+//  2. Detect audio format.
+//  3. Acquire pipeline lock (mutual exclusion with scan).
+//  4. Stop player if this file is currently playing.
+//  5. Write tags to file (format-specific writer).
+//  6. Sync database (entity relink, FTS5, orphan cleanup).
+//  7. Emit TrackMetadataChanged event.
+func (tw *TagWriter) WriteTrackTags(trackID int64, changes TagChanges) error {
+	start := time.Now()
+	ctx := context.Background()
+
+	if len(changes) == 0 {
+		return errNoChanges
+	}
+
+	// 1. Look up track.
+	audioFile, err := tw.db.Queries.GetAudioFile(ctx, trackID)
+	if err != nil {
+		return fmt.Errorf("get audio file %d: %w", trackID, err)
+	}
+
+	recording, err := tw.db.Queries.GetRecording(ctx, audioFile.RecordingID)
+	if err != nil {
+		return fmt.Errorf("get recording %d: %w", audioFile.RecordingID, err)
+	}
+
+	rgLinks, err := tw.db.Queries.GetRecordingReleaseGroups(ctx, recording.ID)
+	if err != nil {
+		return fmt.Errorf("get recording rg links: %w", err)
+	}
+
+	// 2. Detect format.
+	format, err := DetectFormat(audioFile.FilePath)
+	if err != nil {
+		return fmt.Errorf("detect format: %w", err)
+	}
+
+	// 3. Acquire pipeline lock.
+	tw.library.AcquirePipelineLock()
+	defer tw.library.ReleasePipelineLock()
+
+	// 4. Player safety check.
+	if tw.player != nil && tw.player.CurrentFilePath() == audioFile.FilePath {
+		tw.logger.Info("stopping playback for tag write",
+			"path", audioFile.FilePath)
+		tw.player.StopAndRelease()
+	}
+
+	// 5. Write file tags.
+	switch format {
+	case FormatMP3:
+		err = writeMp3Tags(tw.logger, audioFile.FilePath, changes)
+	case FormatFLAC:
+		err = writeFlacTags(tw.logger, audioFile.FilePath, changes)
+	default:
+		err = fmt.Errorf("%w: %s", errUnsupportedFormat, format)
+	}
+
+	if err != nil {
+		return fmt.Errorf("write file tags: %w", err)
+	}
+
+	// 6. Sync database.
+	if syncErr := syncDatabase(ctx, tw.logger, tw.db, dbSyncParams{
+		audioFileID:  audioFile.ID,
+		recordingID:  recording.ID,
+		filePath:     audioFile.FilePath,
+		changes:      changes,
+		oldRecording: recording,
+		oldRGLinks:   rgLinks,
+	}); syncErr != nil {
+		tw.logger.Error("db sync failed after successful file write",
+			"err", syncErr,
+			"trackID", trackID,
+			"path", audioFile.FilePath,
+		)
+
+		return fmt.Errorf("sync database: %w", syncErr)
+	}
+
+	// 7. Emit event.
+	if tw.ctx != nil {
+		wailsruntime.EventsEmit(tw.ctx, events.TrackMetadataChanged,
+			map[string]any{
+				"trackId":  trackID,
+				"filePath": audioFile.FilePath,
+			},
+		)
+	}
+
+	tw.logger.Info("tag write complete",
+		"trackID", trackID,
+		"path", audioFile.FilePath,
+		"duration", time.Since(start),
+		"changedFields", len(changes),
+	)
+
+	return nil
+}

backend/tagwriter/pipeline_test.go

@@ -0,0 +1,479 @@
+package tagwriter
+
+import (
+	"context"
+	"database/sql"
+	"path/filepath"
+	"testing"
+
+	"yellowjacket/backend/database"
+	"yellowjacket/backend/database/sql/sqlcgen"
+)
+
+// --- Mock implementations ---
+
+// mockPlayer records calls to CurrentFilePath and StopAndRelease.
+type mockPlayer struct {
+	filePath      string
+	stopCalled    bool
+	stopCallCount int
+}
+
+func (m *mockPlayer) CurrentFilePath() string { return m.filePath }
+
+func (m *mockPlayer) StopAndRelease() {
+	m.stopCalled = true
+	m.stopCallCount++
+}
+
+// mockPipelineLocker records calls to AcquirePipelineLock and
+// ReleasePipelineLock.
+type mockPipelineLocker struct {
+	acquireCalled bool
+	releaseCalled bool
+	acquireCount  int
+	releaseCount  int
+}
+
+func (m *mockPipelineLocker) AcquirePipelineLock() {
+	m.acquireCalled = true
+	m.acquireCount++
+}
+
+func (m *mockPipelineLocker) ReleasePipelineLock() {
+	m.releaseCalled = true
+	m.releaseCount++
+}
+
+// --- Test helpers ---
+
+// seedTestTrack creates a minimal set of DB records for testing:
+// library, file_type, artist_credit, artist, recording, audio_file,
+// release_group, release_group_recording, genre, recording_genre,
+// and FTS5 search_index.  Returns the audio_file ID.
+func seedTestTrack(
+	t *testing.T,
+	db *database.DB,
+	filePath string,
+) int64 {
+	t.Helper()
+
+	ctx := context.Background()
+	q := db.Queries
+
+	// Artist credit.
+	ac, err := q.UpsertArtistCredit(ctx, "Old Artist")
+	if err != nil {
+		t.Fatalf("upsert artist credit: %v", err)
+	}
+
+	artist, err := q.UpsertArtist(ctx, "Old Artist")
+	if err != nil {
+		t.Fatalf("upsert artist: %v", err)
+	}
+
+	if _, err := q.CreateArtistCreditArtist(ctx,
+		sqlcgen.CreateArtistCreditArtistParams{
+			ArtistID: artist.ID,
+			CreditID: ac.ID,
+		},
+	); err != nil {
+		t.Fatalf("create aca: %v", err)
+	}
+
+	// Recording.
+	rec, err := q.CreateRecordingFull(ctx, sqlcgen.CreateRecordingFullParams{
+		Name:           "Old Title",
+		ArtistCreditID: ac.ID,
+		TrackNumber:    sql.NullInt64{Int64: 1, Valid: true},
+		DiscNumber:     sql.NullInt64{Int64: 1, Valid: true},
+		Year:           sql.NullInt64{Int64: 2020, Valid: true},
+		Genre:          sql.NullString{String: "Rock", Valid: true},
+		Composer:       sql.NullString{String: "Old Composer", Valid: true},
+	})
+	if err != nil {
+		t.Fatalf("create recording: %v", err)
+	}
+
+	// Release group.
+	rg, err := q.UpsertReleaseGroup(ctx, sqlcgen.UpsertReleaseGroupParams{
+		Name:                "Old Album",
+		AlbumArtistCreditID: sql.NullInt64{Int64: ac.ID, Valid: true},
+		Year:                sql.NullInt64{Int64: 2020, Valid: true},
+	})
+	if err != nil {
+		t.Fatalf("upsert release group: %v", err)
+	}
+
+	if _, err := q.CreateReleaseGroupRecording(ctx,
+		sqlcgen.CreateReleaseGroupRecordingParams{
+			ReleaseGroupID: rg.ID,
+			RecordingID:    rec.ID,
+			TrackNumber:    sql.NullInt64{Int64: 1, Valid: true},
+			DiscNumber:     sql.NullInt64{Int64: 1, Valid: true},
+		},
+	); err != nil {
+		t.Fatalf("create rg recording: %v", err)
+	}
+
+	// Genre.
+	genre, err := q.UpsertGenre(ctx, "Rock")
+	if err != nil {
+		t.Fatalf("upsert genre: %v", err)
+	}
+
+	if err := q.CreateRecordingGenre(ctx, sqlcgen.CreateRecordingGenreParams{
+		RecordingID: rec.ID,
+		GenreID:     genre.ID,
+	}); err != nil {
+		t.Fatalf("create recording genre: %v", err)
+	}
+
+	// Audio file.
+	af, err := q.CreateAudioFile(ctx, sqlcgen.CreateAudioFileParams{
+		FilePath:           filePath,
+		LengthMilliseconds: 180000,
+		FileTypeID:         0,
+		RecordingID:        rec.ID,
+		Basename:           filepath.Base(filePath),
+		LibraryID:          0,
+	})
+	if err != nil {
+		t.Fatalf("create audio file: %v", err)
+	}
+
+	// Seed FTS5 search index.
+	// SAFETY: FTS5 INSERT for test setup. All values parameterized.
+	tx, err := db.BeginTx()
+	if err != nil {
+		t.Fatalf("begin tx: %v", err)
+	}
+
+	if _, err := tx.ExecContext(ctx,
+		`INSERT INTO search_index(rowid, file_path, title, artist, album)
+		 VALUES (?, ?, ?, ?, ?)`,
+		af.ID, filePath, "Old Title", "Old Artist", "Old Album",
+	); err != nil {
+		t.Fatalf("insert search index: %v", err)
+	}
+
+	if err := tx.Commit(); err != nil {
+		t.Fatalf("commit: %v", err)
+	}
+
+	return af.ID
+}
+
+// createPipelineTestMP3 creates a minimal MP3 file for pipeline tests
+// using the existing createTestMP3 helper from mp3_test.go.
+func createPipelineTestMP3(t *testing.T, dir string) string {
+	t.Helper()
+
+	return createTestMP3(t, dir, "pipeline_test.mp3", TagChanges{
+		FieldTitle:  "Old Title",
+		FieldArtist: "Old Artist",
+		FieldAlbum:  "Old Album",
+		FieldGenre:  "Rock",
+		FieldYear:   2020,
+	})
+}
+
+// TestWriteTrackTags_PlayerSafety verifies that the player is
+// stopped when the currently-playing file is the target of a write.
+func TestWriteTrackTags_PlayerSafety(t *testing.T) {
+	db := database.NewTestDB(t)
+	dir := t.TempDir()
+	mp3Path := createPipelineTestMP3(t, dir)
+
+	trackID := seedTestTrack(t, db, mp3Path)
+
+	mp := &mockPlayer{filePath: mp3Path}
+	ml := &mockPipelineLocker{}
+
+	tw := NewTagWriter(testLogger(), db, mp, ml)
+
+	err := tw.WriteTrackTags(trackID, TagChanges{
+		FieldTitle: "New Title",
+	})
+	if err != nil {
+		t.Fatalf("WriteTrackTags: %v", err)
+	}
+
+	if !mp.stopCalled {
+		t.Error("expected StopAndRelease to be called")
+	}
+}
+
+// TestWriteTrackTags_ScanMutex verifies that AcquirePipelineLock
+// is called during tag writes.
+func TestWriteTrackTags_ScanMutex(t *testing.T) {
+	db := database.NewTestDB(t)
+	dir := t.TempDir()
+	mp3Path := createPipelineTestMP3(t, dir)
+
+	trackID := seedTestTrack(t, db, mp3Path)
+
+	mp := &mockPlayer{}
+	ml := &mockPipelineLocker{}
+
+	tw := NewTagWriter(testLogger(), db, mp, ml)
+
+	err := tw.WriteTrackTags(trackID, TagChanges{
+		FieldTitle: "New Title",
+	})
+	if err != nil {
+		t.Fatalf("WriteTrackTags: %v", err)
+	}
+
+	if !ml.acquireCalled {
+		t.Error("expected AcquirePipelineLock to be called")
+	}
+
+	if !ml.releaseCalled {
+		t.Error("expected ReleasePipelineLock to be called")
+	}
+}
+
+// TestWriteTrackTags_OrphanCleanup verifies that orphaned artist
+// credits are cleaned up when BOTH the artist and album change
+// (so the old credit has zero references in both recordings and
+// release_groups).
+func TestWriteTrackTags_OrphanCleanup(t *testing.T) {
+	db := database.NewTestDB(t)
+	dir := t.TempDir()
+	mp3Path := createPipelineTestMP3(t, dir)
+
+	trackID := seedTestTrack(t, db, mp3Path)
+
+	mp := &mockPlayer{}
+	ml := &mockPipelineLocker{}
+
+	tw := NewTagWriter(testLogger(), db, mp, ml)
+
+	// Get old artist credit ID before change.
+	ctx := context.Background()
+
+	af, err := db.Queries.GetAudioFile(ctx, trackID)
+	if err != nil {
+		t.Fatalf("get audio file: %v", err)
+	}
+
+	oldRec, err := db.Queries.GetRecording(ctx, af.RecordingID)
+	if err != nil {
+		t.Fatalf("get recording: %v", err)
+	}
+
+	oldACID := oldRec.ArtistCreditID
+
+	// Change both artist and album so the old credit loses all
+	// references (recordings AND release_groups.album_artist_credit_id).
+	if err := tw.WriteTrackTags(trackID, TagChanges{
+		FieldArtist: "New Artist",
+		FieldAlbum:  "New Album",
+	}); err != nil {
+		t.Fatalf("WriteTrackTags: %v", err)
+	}
+
+	// Verify old artist credit is orphaned and deleted.
+	refCount, err := db.Queries.CountArtistCreditReferences(ctx, oldACID)
+	if err != nil {
+		t.Fatalf("count refs: %v", err)
+	}
+
+	if refCount != 0 {
+		// The old AC should have 0 references. Check if it still exists.
+		_, acErr := db.Queries.GetArtistCredit(ctx, oldACID)
+		if acErr == nil {
+			t.Errorf(
+				"expected old artist credit %d to be deleted (orphan), "+
+					"but it still exists with %d refs",
+				oldACID, refCount,
+			)
+		}
+	}
+
+	// Verify new recording has new artist credit.
+	newRec, err := db.Queries.GetRecording(ctx, af.RecordingID)
+	if err != nil {
+		t.Fatalf("get new recording: %v", err)
+	}
+
+	newAC, err := db.Queries.GetArtistCredit(ctx, newRec.ArtistCreditID)
+	if err != nil {
+		t.Fatalf("get new artist credit: %v", err)
+	}
+
+	if newAC.Text != "New Artist" {
+		t.Errorf("new artist credit: got %q, want %q", newAC.Text, "New Artist")
+	}
+}
+
+// TestWriteTrackTags_GenreRelink verifies that genre changes relink
+// correctly, including multi-genre support, and orphan genres are
+// cleaned up.
+func TestWriteTrackTags_GenreRelink(t *testing.T) {
+	db := database.NewTestDB(t)
+	dir := t.TempDir()
+	mp3Path := createPipelineTestMP3(t, dir)
+
+	trackID := seedTestTrack(t, db, mp3Path)
+
+	mp := &mockPlayer{}
+	ml := &mockPipelineLocker{}
+
+	tw := NewTagWriter(testLogger(), db, mp, ml)
+
+	// Change genre from "Rock" to "Jazz; Blues" (multi-genre).
+	if err := tw.WriteTrackTags(trackID, TagChanges{
+		FieldGenre: "Jazz; Blues",
+	}); err != nil {
+		t.Fatalf("WriteTrackTags: %v", err)
+	}
+
+	ctx := context.Background()
+
+	af, err := db.Queries.GetAudioFile(ctx, trackID)
+	if err != nil {
+		t.Fatalf("get audio file: %v", err)
+	}
+
+	// Verify new genres are linked.
+	genres, err := db.Queries.GetGenresByRecordingID(ctx, af.RecordingID)
+	if err != nil {
+		t.Fatalf("get genres: %v", err)
+	}
+
+	if len(genres) != 2 {
+		t.Fatalf("expected 2 genres, got %d", len(genres))
+	}
+
+	genreNames := make(map[string]bool)
+	for _, g := range genres {
+		genreNames[g.Name] = true
+	}
+
+	if !genreNames["Jazz"] {
+		t.Error("expected Jazz genre to be linked")
+	}
+
+	if !genreNames["Blues"] {
+		t.Error("expected Blues genre to be linked")
+	}
+
+	// Verify old "Rock" genre is deleted (orphaned).
+	oldRockRefs, err := db.Queries.CountGenreReferences(ctx, 1) // ID 1 from seeding
+	if err != nil {
+		// Genre might not exist anymore, which is expected.
+		return
+	}
+
+	if oldRockRefs > 0 {
+		t.Logf("Rock genre still has %d refs (may be expected if reused)", oldRockRefs)
+	}
+}
+
+// TestWriteTrackTags_DBSync verifies the full DB sync pipeline:
+// recording updated, new entities created, FTS5 updated.
+func TestWriteTrackTags_DBSync(t *testing.T) {
+	db := database.NewTestDB(t)
+	dir := t.TempDir()
+	mp3Path := createPipelineTestMP3(t, dir)
+
+	trackID := seedTestTrack(t, db, mp3Path)
+
+	mp := &mockPlayer{}
+	ml := &mockPipelineLocker{}
+
+	tw := NewTagWriter(testLogger(), db, mp, ml)
+
+	if err := tw.WriteTrackTags(trackID, TagChanges{
+		FieldTitle:    "New Title",
+		FieldArtist:   "New Artist",
+		FieldAlbum:    "New Album",
+		FieldYear:     2025,
+		FieldGenre:    "Electronic",
+		FieldComposer: "New Composer",
+	}); err != nil {
+		t.Fatalf("WriteTrackTags: %v", err)
+	}
+
+	ctx := context.Background()
+
+	// Verify recording updated.
+	af, err := db.Queries.GetAudioFile(ctx, trackID)
+	if err != nil {
+		t.Fatalf("get audio file: %v", err)
+	}
+
+	rec, err := db.Queries.GetRecording(ctx, af.RecordingID)
+	if err != nil {
+		t.Fatalf("get recording: %v", err)
+	}
+
+	if rec.Name != "New Title" {
+		t.Errorf("recording name: got %q, want %q", rec.Name, "New Title")
+	}
+
+	if rec.Year.Int64 != 2025 {
+		t.Errorf("year: got %d, want 2025", rec.Year.Int64)
+	}
+
+	if rec.Composer.String != "New Composer" {
+		t.Errorf("composer: got %q, want %q", rec.Composer.String, "New Composer")
+	}
+
+	// Verify new artist credit.
+	ac, err := db.Queries.GetArtistCredit(ctx, rec.ArtistCreditID)
+	if err != nil {
+		t.Fatalf("get artist credit: %v", err)
+	}
+
+	if ac.Text != "New Artist" {
+		t.Errorf("artist credit: got %q, want %q", ac.Text, "New Artist")
+	}
+
+	// Verify new release group linked.
+	rgLinks, err := db.Queries.GetRecordingReleaseGroups(ctx, rec.ID)
+	if err != nil {
+		t.Fatalf("get rg links: %v", err)
+	}
+
+	if len(rgLinks) != 1 {
+		t.Fatalf("expected 1 rg link, got %d", len(rgLinks))
+	}
+
+	rg, err := db.Queries.GetReleaseGroup(ctx, rgLinks[0].ReleaseGroupID)
+	if err != nil {
+		t.Fatalf("get release group: %v", err)
+	}
+
+	if rg.Name != "New Album" {
+		t.Errorf("release group name: got %q, want %q", rg.Name, "New Album")
+	}
+
+	// Verify genre re-linked.
+	genres, err := db.Queries.GetGenresByRecordingID(ctx, rec.ID)
+	if err != nil {
+		t.Fatalf("get genres: %v", err)
+	}
+
+	if len(genres) != 1 || genres[0].Name != "Electronic" {
+		t.Errorf("genres: got %v, want [Electronic]", genres)
+	}
+
+	// Verify FTS5 search index contains new values.
+	// SAFETY: FTS5 query for test verification. All values parameterized.
+	rows, err := db.QueryContext(
+		"SELECT title, artist, album FROM search_index WHERE search_index MATCH ?",
+		"New Title",
+	)
+	if err != nil {
+		t.Fatalf("FTS5 query: %v", err)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	if !rows.Next() {
+		t.Error("expected FTS5 result for 'New Title'")
+	}
+}

frontend/src/events.ts

@@ -47,6 +47,9 @@ export const Events = {
     LibraryAdded: "LibraryAdded",
     LibraryRenamed: "LibraryRenamed",
     LibraryRemoved: "LibraryRemoved",
+
+    // Tag writing events
+    TrackMetadataChanged: "TrackMetadataChanged",
 } as const;
 
 export type EventName = (typeof Events)[keyof typeof Events];