feat: implement refund service (#165)

[OSEH-svg]

closes #165

Description

This PR addresses issue #165 by implementing the specialized refundService and updating the transactionService to properly handle refund triggers when a gift goes into a rejected or failed state.

The refund logic now correctly caters to:

• Fiat Transactions (Stripe/Paystack): Dynamically interfaces with the corresponding provider's refund APIs through securely configured HTTP requests and backend wrappers.
• On-chain Transactions (Stellar): Introduces a placeholder reverse transaction back to the centralized custodial signer's platform balance (as individual sender public key secrets are not stored).
• Internal Wallet Balances: Safely reverses database ledgers using the newly added processRefundTransaction method safely located within the transactionService.ts.

All changes adhere strictly to the existing file structure without touching any non-explicitly listed files (maintaining the giftStatusEnum schema constraints by standardizing on exactly representing rejection endpoints as "failed").

Changes Made

• src/server/services/refundService.ts: Introduced the processRefund service method to direct refunds based on paymentProvider origin properties.
• src/server/services/transactionService.ts: Added processRefundTransaction to cleanly and accurately deduct ledger funds from recipients and apply them back to senders.

Verification

• Ran complete workspace compilations without triggering typescript errors inside modified contexts.
• Linter completed specifically with zero errors or warnings inside the modified business logic layer files (src/server/services/refundService.ts & src/server/services/transactionService.ts).

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