fix: prevent race condition on order subtotal calculation

internal/repository/tx_manager.go

@@ -2,6 +2,7 @@ package repository
 
 import (
 	"context"
+	"database/sql"
 
 	"gorm.io/gorm"
 )
@@ -37,3 +38,15 @@ func (m *TxManager) WithTransaction(ctx context.Context, fn func(ctx context.Con
 		return fn(ctxTx)
 	})
 }
+
+// WithTransactionOptions runs fn inside a DB transaction with custom TxOptions (e.g. isolation level).
+func (m *TxManager) WithTransactionOptions(ctx context.Context, opts *sql.TxOptions, fn func(ctx context.Context) error) error {
+	if m == nil || m.db == nil {
+		return fn(ctx)
+	}
+
+	return m.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
+		ctxTx := context.WithValue(ctx, txKey, tx)
+		return fn(ctxTx)
+	}, opts)
+}