fix: assign commit error to err so deferred rollback triggers (closes #125)

When Commit() failed, the error was stored in commitErr instead of err,
so the deferred rollback (which checks err) was skipped.

internal/database/migrations.go

@@ -113,9 +113,9 @@ func (d *Database) applyMigration(ctx context.Context, filename string) error {
 		return fmt.Errorf("failed to record migration: %w", err)
 	}
 
-	commitErr := transaction.Commit()
-	if commitErr != nil {
-		return fmt.Errorf("failed to commit migration: %w", commitErr)
+	err = transaction.Commit()
+	if err != nil {
+		return fmt.Errorf("failed to commit migration: %w", err)
 	}
 
 	return nil