fix(13-02): auto-resolve phantom playlist tracks after library scan

- Add phantom_file_path column to playlist_tracks (migration 7)
- Store original file_path during RemoveLibrary phantom metadata population
- After each successful scan, UPDATE phantom tracks whose phantom_file_path
  now matches an audio_files row, re-linking them and clearing phantom metadata
- Update schema file, sqlc generated code, and database test for new column

backend/database/database.go

@@ -312,6 +312,16 @@ func runMigrations(
 		}
 	}
 
+	// Migration 7: add phantom_file_path to playlist_tracks
+	// for automatic phantom resolution after library re-scans.
+	if version < 7 {
+		if err := migration7PhantomFilePath(
+			ctx, db, logger,
+		); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
@@ -1030,6 +1040,44 @@ func migration6MultiLibrary(
 	return nil
 }
 
+// migration7PhantomFilePath adds the phantom_file_path column to
+// playlist_tracks so that phantom entries can be automatically
+// re-linked to audio_files after a library scan.
+func migration7PhantomFilePath(
+	ctx context.Context,
+	db *sql.DB,
+	logger *slog.Logger,
+) error {
+	logger.Info(
+		"applying migration 7: phantom_file_path column",
+	)
+
+	// SAFETY: ALTER TABLE ADD COLUMN for new nullable column.
+	if _, err := db.ExecContext(ctx,
+		`ALTER TABLE playlist_tracks
+		 ADD COLUMN phantom_file_path TEXT`,
+	); err != nil {
+		if !isDuplicateColumnErr(err) {
+			return fmt.Errorf(
+				"migration 7: could not add column: %w",
+				err,
+			)
+		}
+	}
+
+	if _, err := db.ExecContext(
+		ctx, "PRAGMA user_version = 7",
+	); err != nil {
+		return fmt.Errorf(
+			"could not set user_version to 7: %w", err,
+		)
+	}
+
+	logger.Info("migration 7 complete")
+
+	return nil
+}
+
 // readLibraryDirFromTOML reads the TOML config file and returns
 // the Library.DirectoryPath value, or "" if not configured.
 func readLibraryDirFromTOML(logger *slog.Logger) string {

backend/database/database_test.go

@@ -91,6 +91,7 @@ func TestMigration6FreshDB(t *testing.T) {
 		"phantom_duration_ms":    false,
 		"phantom_genre":          false,
 		"phantom_cover_art_path": false,
+		"phantom_file_path":      false,
 	}
 
 	audioFileIDNullable := false
@@ -171,7 +172,7 @@ func TestMigration6FreshDB(t *testing.T) {
 		t.Error("track_metadata VIEW does not contain library_id")
 	}
 
-	// Verify user_version >= 6.
+	// Verify user_version >= 7.
 	var version int
 
 	verRows, err := db.QueryContext("PRAGMA user_version")
@@ -193,8 +194,8 @@ func TestMigration6FreshDB(t *testing.T) {
 
 	_ = verRows.Close()
 
-	if version < 6 {
+	if version < 7 {
-		t.Errorf("user_version = %d, want >= 6", version)
+		t.Errorf("user_version = %d, want >= 7", version)
 	}
 
 	// Verify libraries table has only the sentinel row on fresh DB.

backend/database/sql/schemas/playlist_tracks.sql

@@ -9,6 +9,7 @@ CREATE TABLE IF NOT EXISTS playlist_tracks (
     phantom_duration_ms INTEGER,
     phantom_genre TEXT,
     phantom_cover_art_path TEXT,
+    phantom_file_path TEXT,
     FOREIGN KEY(playlist_id) REFERENCES playlists(id) ON DELETE CASCADE,
     FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE SET NULL
 );

backend/database/sql/sqlcgen/models.go

@@ -90,6 +90,7 @@ type PlaylistTrack struct {
 	PhantomDurationMs   sql.NullInt64
 	PhantomGenre        sql.NullString
 	PhantomCoverArtPath sql.NullString
+	PhantomFilePath     sql.NullString
 }
 
 type Queue struct {

backend/database/sql/sqlcgen/playlists.sql.go

@@ -16,7 +16,7 @@ INSERT INTO playlist_tracks (
     phantom_title, phantom_artist, phantom_album,
     phantom_duration_ms, phantom_genre, phantom_cover_art_path
 ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
-RETURNING id, playlist_id, audio_file_id, position, phantom_title, phantom_artist, phantom_album, phantom_duration_ms, phantom_genre, phantom_cover_art_path
+RETURNING id, playlist_id, audio_file_id, position, phantom_title, phantom_artist, phantom_album, phantom_duration_ms, phantom_genre, phantom_cover_art_path, phantom_file_path
 `
 
 type AddPlaylistTrackParams struct {
@@ -55,6 +55,7 @@ func (q *Queries) AddPlaylistTrack(ctx context.Context, arg AddPlaylistTrackPara
 		&i.PhantomDurationMs,
 		&i.PhantomGenre,
 		&i.PhantomCoverArtPath,
+		&i.PhantomFilePath,
 	)
 	return i, err
 }

backend/library/crud.go

@@ -244,10 +244,12 @@ func (l *Library) RemoveLibrary(id int64) (*RemovalSummary, error) {
 			phantom_album = sub.album,
 			phantom_duration_ms = sub.duration,
 			phantom_genre = sub.genre,
-			phantom_cover_art_path = sub.cover_art_path
+			phantom_cover_art_path = sub.cover_art_path,
+			phantom_file_path = sub.file_path
 		FROM (
 			SELECT
 				pt.id AS pt_id,
+				af.file_path AS file_path,
 				COALESCE(r.name, '') AS title,
 				COALESCE(ac.text, '') AS artist,
 				COALESCE(rg.name, '') AS album,

backend/library/library.go

@@ -652,7 +652,16 @@ func (l *Library) scanInternal(
 		metrics.OrphanCleanup = time.Since(orphanStart)
 	}
 
-	// --- Phase 6: post-scan variant generation ---
+	// --- Phase 6: resolve phantom playlist tracks ---
+	// Phantom tracks (audio_file_id IS NULL) that have a stored
+	// phantom_file_path matching a newly-scanned audio file are
+	// automatically re-linked.  This handles the case where a
+	// library directory is removed and later re-added.
+	if !cancelled {
+		l.resolvePhantomTracks()
+	}
+
+	// --- Phase 7: post-scan variant generation ---
 	if !cancelled {
 		variantStart := time.Now()
 
@@ -708,6 +717,53 @@ func (l *Library) scanInternal(
 	return metrics
 }
 
+// resolvePhantomTracks re-links phantom playlist_tracks entries
+// whose phantom_file_path now matches an audio_files row.  This
+// runs after every successful scan so that re-adding a previously
+// removed library automatically restores playlist references.
+func (l *Library) resolvePhantomTracks() {
+	// SAFETY: Hand-crafted UPDATE for phantom track resolution.
+	// Matches phantom playlist_tracks (audio_file_id IS NULL,
+	// phantom_file_path IS NOT NULL) against audio_files by
+	// file_path. Clears phantom metadata on resolved rows.
+	// No user input — all values come from the database.
+	result, err := l.db.ExecContext(`
+		UPDATE playlist_tracks SET
+			audio_file_id = (
+				SELECT af.id FROM audio_files af
+				WHERE af.file_path = playlist_tracks.phantom_file_path
+			),
+			phantom_title = NULL,
+			phantom_artist = NULL,
+			phantom_album = NULL,
+			phantom_duration_ms = NULL,
+			phantom_genre = NULL,
+			phantom_cover_art_path = NULL,
+			phantom_file_path = NULL
+		WHERE audio_file_id IS NULL
+			AND phantom_file_path IS NOT NULL
+			AND EXISTS (
+				SELECT 1 FROM audio_files af
+				WHERE af.file_path = playlist_tracks.phantom_file_path
+			)`)
+	if err != nil {
+		l.logger.Warn(
+			"could not resolve phantom playlist tracks",
+			"err", err,
+		)
+
+		return
+	}
+
+	resolved, _ := result.RowsAffected()
+	if resolved > 0 {
+		l.logger.Info(
+			"resolved phantom playlist tracks",
+			"count", resolved,
+		)
+	}
+}
+
 // progressInterval controls how often scan progress events are
 // emitted to the frontend.
 const progressInterval = 300 * time.Millisecond