MS-170/feat: Add Gift Card and Voucher Refund Support

Summary

Implements comprehensive refund functionality for gift cards and vouchers.

Changes

• Email Templates: 6 new refund confirmation templates
• API Endpoints: 10 new methods for gift card/voucher operations
• Data Model: Enhanced RefundOrder with 10 tracking fields
• Enums: Extended RefundMethod with GIFT_CARD and VOUCHER

Impact

• Enables alternative refund methods beyond traditional payment processors
• Provides complete audit trail for gift card/voucher refunds
• Supports partial and progressive refund scenarios