feat(10-01): implement migration 6 and pre-migration backup

- Add backupDatabase() for timestamped .db file backup before migration
- Add migration6MultiLibrary() with all 14 steps: FK OFF, create libraries table,
  insert default library from TOML config, add library_id to audio_files, rebuild
  playlist_tracks with SET NULL FK and 6 phantom columns, backfill phantom metadata,
  recreate track_metadata VIEW with library_id, FK ON, clean TOML config
- Add readLibraryDirFromTOML() and removeLibraryDirFromTOML() helpers
- Update runMigrations signature to accept dbPath for backup
- Add sentinel library row in NewTestDB for FK constraint satisfaction

backend/database/testhelper.go

@@ -57,10 +57,19 @@ func NewTestDB(t *testing.T) *DB {
 		}
 	}
 
-	if err := runMigrations(ctx, db, slog.Default()); err != nil {
+	if err := runMigrations(ctx, db, slog.Default(), ":memory:"); err != nil {
 		t.Fatalf("could not run migrations: %v", err)
 	}
 
+	// Insert a sentinel library row at id=0 so audio_files inserts
+	// using the DEFAULT library_id=0 satisfy the FK constraint.
+	if _, err := db.ExecContext(
+		ctx,
+		"INSERT INTO libraries (id, name, path) VALUES (0, 'Test', '/test')",
+	); err != nil {
+		t.Fatalf("could not insert test library: %v", err)
+	}
+
 	queries := sqlcgen.New(db)
 
 	t.Cleanup(func() { _ = db.Close() })