feat(06-01): create track_metadata VIEW schema and migration 4

- Add track_metadata_view.sql for sqlc VIEW awareness
- Add migration4TrackMetadataView for existing databases
- sqlc generate produces TrackMetadatum model from VIEW
- migration2 inline JOIN preserved (runs before VIEW exists)

backend/database/database.go

@@ -276,6 +276,15 @@ func runMigrations(
 		logger.Info("migration 3 complete")
 	}
 
+	// Migration 4: create track_metadata VIEW.
+	if version < 4 {
+		if err := migration4TrackMetadataView(
+			ctx, db, logger,
+		); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
@@ -379,6 +388,76 @@ func migration2BasenameAndFTS(
 	return nil
 }
 
+// migration4TrackMetadataView creates the track_metadata VIEW that
+// consolidates the 5-table JOIN used by FTS5 search queries.
+// Fresh databases get the VIEW from the embedded schema file;
+// this migration covers databases created before the VIEW existed.
+func migration4TrackMetadataView(
+	ctx context.Context,
+	db *sql.DB,
+	logger *slog.Logger,
+) error {
+	logger.Info(
+		"applying migration 4: track_metadata VIEW",
+	)
+
+	if _, err := db.ExecContext(ctx, `
+		CREATE VIEW IF NOT EXISTS track_metadata AS
+		SELECT
+			af.id,
+			af.file_path,
+			af.length_milliseconds,
+			COALESCE(r.name, '') AS title,
+			COALESCE(ac.text, '') AS artist_name,
+			r.track_number,
+			r.disc_number,
+			COALESCE(rg.name, '') AS album,
+			CAST(COALESCE(
+				(SELECT GROUP_CONCAT(g.name, '||')
+				 FROM recording_genres rg_sub
+				 JOIN genres g ON rg_sub.genre_id = g.id
+				 WHERE rg_sub.recording_id = r.id),
+				''
+			) AS TEXT) AS genre,
+			COALESCE(r.year, 0) AS year,
+			COALESCE(r.composer, '') AS composer,
+			COALESCE(ft.extension, '') AS file_type,
+			af.sample_rate,
+			af.bit_depth,
+			af.channels,
+			af.bitrate,
+			af.file_size
+		FROM audio_files af
+		LEFT JOIN recordings r ON af.recording_id = r.id
+		LEFT JOIN artist_credit ac ON r.artist_credit_id = ac.id
+		LEFT JOIN (
+			SELECT recording_id,
+				MIN(release_group_id) AS release_group_id
+			FROM release_group_recordings
+			GROUP BY recording_id
+		) rgr ON r.id = rgr.recording_id
+		LEFT JOIN release_groups rg ON rgr.release_group_id = rg.id
+		LEFT JOIN file_types ft ON af.file_type_id = ft.id
+	`); err != nil {
+		return fmt.Errorf(
+			"migration 4: could not create track_metadata VIEW: %w",
+			err,
+		)
+	}
+
+	if _, err := db.ExecContext(
+		ctx, "PRAGMA user_version = 4",
+	); err != nil {
+		return fmt.Errorf(
+			"could not set user_version to 4: %w", err,
+		)
+	}
+
+	logger.Info("migration 4 complete")
+
+	return nil
+}
+
 // isDuplicateColumnErr returns true when the error is SQLite's
 // "duplicate column name" error from an ALTER TABLE ADD COLUMN
 // on a column that already exists.